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Irradiation effects in Ultrahigh Molecular Weight Polyethylene

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Irradiation effects in Ultrahigh Molecular Weight Polyethylene

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Content IRRADIATION EFFECTS IN ULTRAHIGH MOLECULAR WEIGHT POLYETHYLENE by Ashok N. Shinde A Thesis Presented to the FACULTY OF THE SCHOOL OF ENGINEERING UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE IN CHEMICAL ENGINEERING May 1985 UMI Number: EP41821 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI EP41821 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Ch ’ 85 SS56 3oysQ- v/ This thesis, w ritte n by Ashok Shinde under the guidance of his F a cu lty Comm ittee and approved by a ll its members, has been presented to and accepted by the School of E ngineering in p a rtia l fu lfillm e n t o f the re quirements fo r the degree of Masters of Science of Chemical Engineering fA r rih 2S , 1^ 3 5 F a cu lty C om m ittee f ) To my loving parents iii ACKNOWLEDGEMENTS The author expresses his sincere gratitude to Dr. Ronald Salovey for his continual guidance and advice during the course of this research. He wishes to thank him and the Chemical Engineering Department for providing the financial support. He also feels indebted to Dr. W. V. Chang and Dr. K. Shing for their helpful discussions and comments throughout the course of this research. He takes this opportunity to thank his colleagues Hemant Gupta and Khushroo Lakdawala for their encouragement and helpful discussions. He also thanks Parker Seal Company for permitting the usage of their DSC and IR spectrophotometer. Finally, the author wishes to express his sincere appreciation to the Chemical Engineering Department for providing a friendly and intellectually stimulating environment. iv TABLE OF CONTENTS Page DEDICATION ii ACKNOWLEDGEMENTS iii LIST OF FIGURES vi LIST OF TABLES. vii ABSTRACT viii CHAPTER I. INTRODUCTION 1 II. EXPERIMENTAL 9 2.1 Materials 9 2.2 Sample Preparation 9 2.3 Irradiation of Samples 12 2.4 Differential Scanning Calorimetry 12 2.5 Infrared Spectroscopy 13 III. RESULTS 15 3.1 DSC Analysis 15 3.2 Heat of Fusion 15 3.3 Melting Temperatures 29 3.4 Solubility 30 3.5 Infrared Spectroscopic Analysis 30 IV. DISCUSSION 32 V. CONCLUSIONS 42 V REFERENCES 47 APPENDIX 51 Tables of Results vi LIST OF FIGURES Page FIGURE 1. Solubility Changes in Irradiated Polyethylene 4 2. Effect of radiation dose on the heat of fusion (per unit mass of specimen or crystalline phase) 6 3* Irradiation effects in UHMWPE Heat of Fusion 16 4. Irradiation effects in UHMWPE Peak Melting Temperature 18 5. Irradiation effects in UHMWPE Carbonyl Content 19 6. Irradiation effects in UHMWPE trans-Vinylene Content 20 DSC ANALYSIS (Individual Samples) 7. UHMWPE - HE 1900 Compression Molded, Slowly Crystallized 21 8. UHMWPE - HE 1900; Compression Molded, Quenched 22 9. UHMWPE - HE 1900; Melted in Nitrogen Before Irradiation 23 10. UHMWPE - Extruded 24 11. UHMWPE - HE 1900; Powder in Oxygen 25 12. UHMWPE - HE 1900; Powder in Nitrogen 26 13. HDPE; Slowly Crystallized 27 14. HDPE; Quench'Crystallized 28 vii LIST OF TABLES Page TABLE . 1. Table of Samples 14 RESULTS OF DSC ANALYSIS 2. HE-1900; [CM, SC] 52 3. IE-1900; [CM, QC] 53 4. HE-1900; [NITROGEN] 54 5. HE-1900; [VACCUUM] 55 6. EXTRUDED UHM 56 7. HE-1900; [Oxygen, P] 57 8. HE-1900; [NITRO, P] 58 9. HDPE; [SC] 59 0, HDPE; [QC] 60 viii ABSTRACT Physical properties of semicrystalline polymers are significantly governed by their state of aggregation. The crystallinity and the morphology of the polymer are its important components. Ionizing radiation such as gamma-radiation and high energy electron beam may remarkably alter this state of aggregation under certain conditions. UHMWPE is one polymer which is found to be quite sensitive to the ionizing radiation. In these studies, UHMWPE samples, varying in crystallinity, were prepared and irradiated in different environments. The studies reveal that the polymer undergoes substantial crystallinity changes at radiation doses as low as 5 Mrad. It is further found that these changes are, to some extent, independent of the environment during irradiation. However, the high crystallinity UHMWPE has exibited a remarkable effect of the environment during irradiation on its crystallinity changes. It is attempted to use these unusual observations as important clues to the morphological subtleties of this commercially important polymer. 1 CHAPTER I INTRODUCTION The exposure of polymers to high energy radiations, like gamma rays and high energy electrons, is known to.induce numerous physical and chemical changes in them. Gamma rays and electrons transfer the energy primarily by collisions with electrons in the material. Once the energy is deposited at any single electron in the material, a large number of very rapid processes may occur. If the energy is sufficient, the electron is expelled from its orbit, leaving a positive ion. This ion is probably in an excited state and may be unstable, thereby rapidly dissociating into free radicals or molecular fragments (1). Subsequently, the free radicals may react in a number of different ways depending upon the "environment" and temperature. The resulting reactions manifest themselves into either crosslinking, chain scission or changes in unsaturation. As a consequence of these chemical changes the physical properties of the polymer are also affected depending upon the extent of these changes. The effects are readily measurable through the changes in solubility, crystallinity, mechanical properties, surface properties etc. Even though most of these changes persist in almost all polymers, their extent is significantly governed by a large'number of factors. The chemical nature of the polymer is by far the most important one. Polymers such as polyoxymethylene primarily undergo chain scission whereas polyethylene is crosslinked by irradiation (2). Other factors include molecular weight of the polymer and its distribution (3), morphology and crystallinity (4), temperature and environment during irradiation etc. Amorphous and crystalline polymers differ radically in their response to radiation exposure. This difference is primarily due to the caging effect of crystalline regions on the fate of free radicals produced. The consequences of this anisotropy were demonstrated in semicrystalline polymers (4). It was shown that the post-irradiation reactions are not random, even though the primary event of formation of activated species (normally a free radical) is uniform throughout the material. Abundant experimental evidence has shown that crosslinkinng takes place primarily in the fold surface of polyethylene single crystals or in the non-crystalline phase of bulk polymer. The scarcity of crosslinks inside the crystal lattice* at relatively low irradiation doses has been ascribed to carbon atoms on adjacent e chains being too far apart (4.1 A atleast) and to the lattice being too rigid at room temperature for the interchain C-C bonds O (1.5 A) to form easily. Other chain configurations like in extended chain morphology also markedly affect the irradiation behaviour of polyethylene (5). 3 Polyethylene is one of the most extensively" studied polymers, primarily because of the simplicity of its chemical structure. Later these studies emerged to be of considerable commercial significance especially in crosslinking of this polymer to increase its structural stability under the influence of mechanical stresses. A large number of excellent studies (1,6,7,8,9) have been published eventually in the last two decades. It was generally observed that solubility of the polymer decreased monotonically with the radiation dose (Fig.1).The effiency of gel formation was found to be a strong function of the morphology of the polymer, its crystallinity, temperature of irradiation and so on. Out of the many chemical changes, the vinyl type unsaturation, initially present in the material, was found to decrease rapidly upon irradiation. The trans-vinylene unsaturation, on the other hand, increased steadily with the radiation dose. Its dependence on the radiation dose was observed to be linear especially at low doses. The formation of this species was also found to be independent of the crystallinity and temperature (10) as well as the environment during irradiation. Crystallinity of the polymer was hardly affected upto about 200 Mrad., but its destruction at very high doses (about 2000 Mrad) was observed by Keller and others (11,12). In most of these observations the influence of chain scission was quite inconsequential as compared to that of crosslinking. 4 Figure 1 Solubility Changes in Irradiated Polyethylene 0.8 • - Af * 122,000 « - M = 26,000 o-M - 17,800 0 -/1 ^= 7,400 2,800 0.6 » 0 .2 I 2 3 4 5 7 6 13 12 14 Plot of gel fraction against R/Rc for indicated molecular weight fractions of linear polyethylene irradiated in melt at 133C C 0.8 0.6 04 - 0.2 0.0 m- u « 1 2 2 , 0 0 0 77,000 o- = *6,000 — D- 26,000 4- *2 = 30,000 H r o r o $ e n e > e d 4- 50,000 H j f o r o p r n o t e d - Kr 30,000 V- A 7 * v *v 50,000 Plot of gel fraction against R/Rc fo r indicated molecular weight fractions of linear polyethylene irradiated in crystalline state at 133°C 5 In the light of these observations the increase in crystallinity upon irradiation, as first reported by Roe (13)» in Ultrahigh Molecular Weight Polyethylene (UHMWPE) was very intriguing. UHMWPE is currently the polymer often used in orthopaedic prostheses. Its primary application has been in the I acetabular cup component of the total hip replacement. This use has resulted mainly from the good abrasion resistance and biocompatibility exibited by UHMWPE. In the recent studies by Bhateja et al.(5,14,15,16) a rapid f I increase in the tensile modulus (effect of increase in i j | crystallinity) at low radiation doses followed by an almost i : linear increase (effect of crosslinking in the amorphous phase) i t | was found (14). The former result is in accordance with the ! crystallinity changes previously reported (13) and also observed by this group (Fig.2). Surprisingly these crystallinity effects were found to be time dependent, increasing monotonically over a long period of time subsequent to irradiation. UHMWPE, with enhanced crystallinity obtained by crystallization at high temperatures and pressures, also exibited this crystallinity effect upon irradiation.(5). Based on these observations it was speculated that these changes are manifestations of the predominance of chain scission in this material. UHMWPE is only 50% crystalline because of its high molecular weight. Large number of tie molecules may exist 6 80 70 < CL £ 60-} -J \T < u X O a 40 40 80 Dose (MRAD) 120 160 Fig, 2. Effect of radiation dose on the heat of fusion (per unit mass of specimen or crystalline phase) of all three materials: (o) UHMWPE A , (•) UHMWPE B , (®) HDPE. 7 as a consequence (14). Tie molecules are chains which are incorporated in more than one crystal and serve to hold them together. It is suggested that these chains rupture on exposure to ionizing radiation permitting them to fold and crystallize. Relaxation of the chains, thereby reducing the number of defects in the crystallites, may be responsible for the increase in melting temperatures (14). However the gradual decrease in crystallinity, found by Bhateja (14) (Fig.2) above a certain radiation dose cannot be justified by the above speculation. Our studies were aimed at re-evaluating the previous observations; those made by us as well as others, and provide further experimental proof of this unusual irradiation effect. In addition to detecting the crystallinity effects, it was thought necessary to look into the concomitant chemical changes by IR spectroscopy. The effect of environment during irradiation is known to differ significantly . Different environments i.e. oxygen rich and inert, were chosen in our studies to elucidate the role of oxygen in the irradiation behaviour of UHMWPE. In addition, samples with crystallinities varing from 40 to 80f» were prepared to study the influence of crystallinity on the irradiation response. In the later part of these studies, attention was drawn upon a very strange observation. In DSC analysis this phenomenon occurs as a decrease in specific heat of the polymer. This was 8 also previously reported by Zachariades et al (17)- They found that the anisotropy in UHMWPE powder (seen under cross-polarized optical microscope) exists even at 220°C (which is well above the melting temperature of polyethylene) in UHMWPE powder. The decrease in specific heat occurs in the vicinity of this temperature. This phenomenon does not take place in bulk UHMWPE samples. Many authors have studied the mechanical behaviour of. UHMWPE. A highly non-linear response, in creep, was found (16,18). A one dimensional constitutive equation was developed by Zapas et al (18) which describes quantitatively the creep and recovery behaviour in unaxial extension of UHMWPE in the region of small deformations. The agreement between calculated and experimental response is excellent. Bhateja (16) has shown that the creep behaviour of UHMWPE can be improved by irradiation, evidently due to networking of the polymer. CHAPTER II EXPERIMENTAL TECHNIQUE 2.1 MATERIAL The polymer used in this study is the commercial grade high molecular weight polyethylene (termed Ultrahigh Molecular Weight Polyethylene, UHMWPE) obtained from Hercules Inc (Hercules 1900). As received, the raw polymer was in the form of powder. It was confirmed later from the manufacturer that it contained no additives such as antioxidant or stabilizers. The polymer has a molecular weight of 4x106 according to the specifications of the manufacturer based on intrinsic viscosity measurements made in decalin. 2.2 SAMPLE PREPARATION Compression Molded Samples These samples were prepared by compression molding the powder into circular discs of about 1 mm. thickness in a Preco press. This thickness was chosen to ensure complete melting and fusion of the polymer particles. The o temperature of the plates was raised to 220 C. Then 2200 10 kg. platen pressure was applied for about 5 min., after which the pressure was raised to 9000 kg. The plates were water-cooled to room temperature after 5 min. The sample was then remelted in the oven under vacuum at 180°C. For slowly crystallized samples the oven was allowed to cool gradually to room temperature. For quench crystallized samples the molten specimens were immediately dropped into a pool of liquid nitrogen. Sealed Samples i) Vacuum The powder was placed in a glass ampule which was subsequently evacuated. The ampule was constantly ' ’stirred" to get rid of any trapped air in the powder. The mouth of the ampule was then sealed while the vacuum was still on. Some of the samples made as above were melted in the oven under vacuum at 180°C. The UHMWPE powder melts at this temperature (as detected in DSC by the large decrease in crystallinity from 79 % to 50 % upon remelting) but the particles do not fuse (as the polymer does not flow due the high viscosity of the melt). The oven was then allowed to gradually cool to room temperature. ii) Nitrogen Vacuum was applied to remove the trapped air in the 11 ampules containing UHMWPE powder. Using a three-way valve nitrogen was passed into the ampule. Vacuum was applied again. Finally, nitrogen was passed into the ampule and its mouth sealed. Samples, made as above, were then melted in the oven under vacuum at 180°C, which was then cooled gradually to room temperature. iii) Oxygen The procedure is same as above except that the powder was sealed in oxygen rich atmosphere. Extruded Samples Circular discs of about 5 mm thickness were cut out of an extruded bar of UHMWPE (~8 cm diameter) which was obtained as such (19). These were then melted in the oven under vacuum at 180°C. Subsequently the oven was allowed to cool gradually to room temperature. High Density Polyethylene About 1 cm thick mold was chosen. The pellets (obtained from Dow Chemical Company) placed in the mold, were heated to about 170°C. The platen pressure was then gradually raised to 9000 kg. After 5 min. the plates were water-cooled to room temperature. All the necessary samples for one set were made in the same mold. For the quenched crystallized samples, the hot mold at 170°C was dropped in 12 to a pool of cold water. 2.3 IRRADIATION OF SAMPLES The samples were irradiated by Radiation Sterilizers Inc., Tustin. Cobalt-60 was used as a source of gamma radiation. The radiation doses were delivered at a rate of 0.15 Mrad/hr. Samples received doses of 2.5, 5-0, 10.0 and 20.0 Mrad. The temperature rise during irradiation was reported to be only 2 to 3°C since the dose rate was small. The doses delivered to the samples were measured using "Transfer Dosimetry technique" (20) by Radiation Sterilizers Inc. 2.4 DIFFERENTIAL SCANNING CALORIMETRY About 5 to 10 mg. sample was used for calorimetric measurements in the differential scanning calorimeter (PERKIN-ELMER DSC 2). The sample was melted at a heating rate of 20°C/min (1st melting). It was then crystallized at a cooling rate of 20°C/min. It was remelted (Ilnd melting) at the same heating rate. To ensure accuracy of the data acquired, indium and lead standards were used to apply the necessary corrections for the baseline. Very small deviations, only in temperatures, were observed. The unirradiated sample was tested frequently to 13 assure the reproducibility of the data. 2.5 INFRARED SPECTROSCOPY PERKIN-ELMER 1330 Infrared Spectrophotometer was used for the Infrared analysis. The spectrum was obtained at a scanning rate of 316 cm'1 /min. Attempts to use even smaller scanning rates were unsuccessful because of excessive noise in the response. The absorbance at 1376 cm" (corresponding to the asymmetric vibration of C-H bond in methyl group) which is known to remain unaffected upon irradiation (21), was selected for internal calibration. Film thickness was also measured for the computation of G values. SAMPLE PREPARATION: Thin films (0.125 to 0.25 mm thick) were prepared by compression molding small amounts of sample at 220°C and 10000 kg. platen pressure. Films of uniform thickness were obtained using this method. TABLE 1 TABLE OF SAMPLES + = _ = — r = _ = = = — =4 1 | I 1 1 1 Nomenclature 1 i i I Description I i 1 CM,SC 1 1 1 | | HE-1900 compression molded, slowly . I crystallized, irradiated in air. I I 1 CM,QC 1 1 1 | | HE-1900 compression molded, quench | crystallized, irradiatted in air. I I i 1 I NITROGEN I 1 1 I I HE-1900 melted, slowly crystallized | and irradiated in nitrogen atmosphere. I I 1 1 I VACUUM 1 1 1 1 | HE-1900 melted, slowly crystallized | and irradiated in vacuum. I I 1 1 1 Extruded I I UHMWPE 1 1 I Extruded UHMWPE, melted, slowly I crystallized and irradiated in air. | I 1 1 I OXYGEN,P i 1 1 | f HE-1900 powder sealed and irradiated I in oxygen atmosphere. I j 1 NITRO,P 1 1 1 | | HE-1900 powder sealed and irradiated I in nitrogen atmosphere. I I I HDPE,SC 1 1 1 1 | High density polyethylene, slowly crys- I tallized and irradiated in air. I I 1 HDPE,QC 1 1 1 1 1 I High density polyethylene, quench crys- I tallized and irradiated in air. I I 15 CHAPTER III RESULTS 3.1 DSC ANALYSIS The heats of fusion, crystallinities and melting tempe ratures obtained for various samples are tabulated (Table 2-12). A value of 69 cal/gm was used for 100$ crystalline polyethylene (22). The degree of crystallinity was calculated from the heat of fusion data as : (heat of fusion of sample) $ Crystallinity = ----------- ----------------------------- (heat of fusion of 100$ crystalline PE) The peak temperatures of the melting endotherm obtained in DSC are taken to be the melting temperatures of the polymer. The thermal data on both first melting and second melting are given. 3.2 HEAT OF FUSION The heat of fusion of almost all the samples was found to increase with the irradiation dose (Fig.3). Exceptions to this were the UHMWPE powder irradiated in nitrogen atmosphere and HDPE HEAT O F FUSION (Cal/gm) 16 Figure 3 IRRADIATION EFFECTS IN UHMWPE H eat o f f u s io n 100 60 50 .. A 60 ---9L --<) o- .. 50 30 ./ 0 20 10 RADIATION DOSE (Mrad) • CM, SC x M e lte d in n i t r o g e n o CM, QC + Oxygen, P A E xtru ded UHM * N i t r o g e n , P PERCENT CRYSTALLINITY 17 (Fig.12,13). Variable degrees of increases are obtained depending upon the original crystallinity of the polymer and the environment during irradiation. Largest increase (10 cal/gm) was obtained in the bulk samples (~50-60% crystallinity): compression molded, slowly crystallized and quench crystallized UHMWPE and extruded UHMWPE irradiated in air. UHMWPE samples irradiated in an inert atmosphere also showed an unprecedented increase in the heat of fusion (Fig.3). However, this rise (6 cal/gm) is relatively less than in the other bulk samples. The heat of fusion on second melting was found to decrease more or less, with the irradiation dose in all the samples. However, at low doses (upto 5 Mrad) the bulk UHMWPE samples showed either a slight increase or a constant value of the heat of fusion. At higher doses the heat of fusion decreased for all the bulk UHMWPE samples (Fig.7-10). The largest decrease (-10 cal/gm) was obtained in HDPE, slowly crystallized before irradiation. The UHMWPE powder showed an entirely different variation. While the heat of fusion increased with irradiation dose in the oxygen environment (Fig.11), an unprecedented decrease was observed in the nitrogen atmosphere (Fig.12). These changes are, however, very subtle (2-3 cal/gm) in comparison to those obtained in bulk UHMWPE samples. It is interesting to note that originally the UHMWPE powder has a higher crystallinity (~79%) PEAK MELTING TEMPERATURE ( C) 18 Figure 4 IRRADIATION EFFECTS IN UHMWPE Peak m e l t i n g te m p e r a tu r e 160 . . 150 .. — #5 .o o- 120 15 0 5 10 20 RADIATION DOSE (Mrad) • CM, SC x M e lte d in n it r o g e n o CM, QC + Oxygen, P A E xtru d e d UHM * N i t r o g e n , P Figure 5 IRRADIATION EFFECTS IN UHMWPE C arbonyl c o n te n t Ni t r o g e n , P m e lte d in s e a le d tube a f t e r i r r a d i a - t i on ^ E x tru d e d UHM x M e lte d in n it r o g e n + O xygen, P * N i t r o g e n , P o C N / 0.8- o >- 0.6- o 0.2- A -XT 20 0 15 10 RADIATION DOSE (Mrad) F ig u re 6 IRRADIATION EFFECTS IN UHMWPE t r a n s - V i n y l e n e c o n te n t x M e lte d in n it r o g e n ^ Extru ded UHM * * N i t r o g e n , P 0.12 < 0 .1 0 " < o LU ^ 0.08" LU Z LU >- I t n c r o 0.06 " LU > I — <t _J LU A 0.02 " 0 5 10 20 RADIATION DOSE (Mrad) HEAT O F FUSION (Cal/gm) 21 Figure 7 UHMWPE - HE 1900 Compression m olded, s lo w ly coo led 100 60 •■80 50 ■ ■70 ..60 A0 ..50 30 15 5 10 20 0 RADIATION DOSE (Mrad) • First me 11ing o Second melting PERCENT CRYSTALLINITY HEAT O F FUSION ( Cal/gm) 22 Figure 8 UHMWPE - HE 1900 Compression m e ld ed , quenched 1100 60. . -80 50.. - 7 0 ..60 '•50 .40 20 RADIATION DOSE (Mrad) • First melting o Second melting PERCENT CRYSTALLINITY HEAT O F FUSION (Cal/gm) 23 Figure 9 UHMWPE - HE 1900 M e lte d in n i t r o g e n b e f o r e i r r a d i a t i o n .. 90 60. . 50.. .. 50 30.. •• A0 20 15 0 5 10 RADIATION DOSE (Mrad) • First me 1t i ng o Second melting PERCENT CRYSTALLINITY HEAT O F FUSION (Cal/gm ) 24 Figure 10 UHMWPE - E x tru d e d 100 .. 90 60 . . ..80 T 70 ..50 30 .. 15 20 10 0 RADIATION DOSE (Mrad) • First melting o Second melting PERCENT CRYSTALLINITY HEAT O F FUSION (Cal/gm) 25 Figure 11 60 - • 50 -- 40 - 30 -■ UHMWPE - HE 1900 Powder in oxygen RADIATION DOSE (Mrad) -•100 - 9 0 • Fi rst melting o Second melting PERCENT CRYSTALLINITY HEAT O F FUSION (Cal/gm) 26 Figure 12 UHMWPE - HE 1900 Powder in n i t r o g e n RADIATION DOSE (Mrad) • First me 11 i ng o Second me 11 i ng PERCENT CRYSTALLINITY HEAT O F FUSION (Cal/gm) 27 60 . . 50 kO . . 30 .. Figure 13 HDPE S lo w ly c r y s t a l l i z e d 5 10 15 RADIATION DOSE (Mrad) • First melt i ng o Second melting 1 100 - 90 -80 - 70 ■ © 60 - 50 PERCENT CRYSTALLINITY HEAT O F FUSION (Cal/gm) 28 Figure 14 HDPE Quench c r y s t a l l i z e d .. 100 10 15 RADIATION DOSE (Mrad) • Fi rst melt i ng o Second melting PERCENT CRYSTALLINITY 29 than the bulk samples. It was also found that the heat of fusion of UHMWPE in bulk form continues to increase with the passage of time, subsequent to irradiation, for all the doses. This ’ 'aging'1 phenomena seems to be a common feature to all the samples with 50% crystallinity. These changes are found to be appreciably large (3-4 cal/gm). 3.3 MELTING TEMPERATURES Melting temperatures for all the bulk samples were found to increase with the radiation dose (Fig.4). The extent of this increase was governed by the original crystallinity of the polymer and the irradiation environment. However, UHMWPE powder behaved quite differently. Astonishingly, in both oxygen and nitrogen environment, the melting temperatures dropped monotonically with the radiation dose (Fig.4). This effect is exactly opposite to that found in all other samples. On second melting the peak melting temperatures were found to decrease with the radiation dose. Some samples showed slight increases at low dose but the temperatures ultimately begin to drop at higher doses. Most remarkable melting point change on second melting is found in the UHMWPE powder. A drastic decrease from 146.7°C to 130.7 °C is observed upon second melting of 30 unirradiated UHMWPE powder. 3.4 SOLUBILITY Preliminary solubility studies in boiling xylene indicate that UHMWPE powder is completely soluble but the solution gels on cooling to room temperature. However, it was found that the irradiated UHMWPE powder (20 Mrad) not only dissolves readily but on cooling the solution it forms white flaky precipitate in the solution. Similar suspension of single crystals was obtained in dilute solutions of HDPE in the same solvent. 3.5 INFRARED SPECTROSCOPIC ANALYSIS Two things are primarily focussed on in these studies. A a) trans-vinylene absorption at 965 cm and A b) carbonyl (C-Q) absorption at 1720 cm" . The yields of these two groups are calculated taking the -1 absorption at 1376 cm , corresponding to the asymmetric vibrations of methyl group, as an internal standard. This group is reported to remain unaffected upon irradiation (21). The trans-vinylene absorption is observed to increase linearly with the radiation dose (Fig.6). Apparently there does not seem to be any effect of the irradiation environment on the formation of 31 this species. However, the yields of trans-vinylene. in UHMWPE powder irradiated in nitrogen atmosphere are surprisingly higher than the bulk UHMWPE samples (Fig.6). There is a relatively large amount of carbonyl formation at 2.5 Mrad dose for all the bulk UHMWPE samples. Otherwise it increases more or less, with the radiation dose (Fig.5). At higher doses (about 20 Mrad) the amounts of carbonyl groups tend to reach a saturation in all samples. 32 CHAPTER IV DISCUSSION On the basis of the preliminary observations on the crystallinity changes in irradiated UHMWPE two very fundamental processes have been realised. For the first time in polyethylene, chain scission seems to have superceded its counterpart i.e. crosslinking. Besides, our studies reveal that this process is, to some extent, independent of the environment during irradiation. The increase in crystallinity of bulk UHMWPE has been attributed to the scission of tie molecules (14) permitting their crystallization on the existing substrate. Upon exposure to ionizing radiation, scission of the C-C bond can occur irespective of the irradiation environment by an ionic mechanism (1). However, from the studies of thermo-oxidative degradation of polyehtylene (23,24) it is well established that oxygen greatly enhances this process. Oxygen is known to be the best free radical scavenger. The reaction between the two may follow the mechanism: 33 hv R'__CHZ— R' ' R>__CH— R' ' + H > 0 0 R'— CH— R* ' (hydroperoxy radical) 0 0 R'— CH— R' ' > Rearrangements / > Hydroperoxide where R' and R'' are — (CH2)„— . Out of the many precursors of the carbonyl group, hydroperoxides are known to be the most stable ones at room temperature but they decompose at higher temperatures. Scission of the chain at the site of the hydroperoxide group resulting in a terminal carbonyl group is possible for instance according to the following mechanism: H 0 0 0 — CH2— CH— CH*— = = ==> — CH2— CH— CH2— + .OH 0 0 I — CHj— CH— CH2— = = ==> — CHZ— CH + .CH*-- Many other mechanisms are possible to illustrate the formation of a carbonyl group from the hydroperoxy radical or the hydroproxide group (23). Most of these reactions result in the formation of a carbonyl group at the end of the severed chain. This implies 34 that the number of chain scissions should be proportional to the amount of carbonyl groups formed. Our studies revealed different carbonyl contents in all samples depending upon the environment during irradiation and the initial crystallinity. The highest carbonyl yield was obtained in UHMWPE powder irradiated in oxygen atmosphere(G ~ 25). In all the samples the amount of carbonyl was found to increase monotonically with the radiation dose, tending to reach saturation at higher radiation dose (20 Mrad). This is in accordance with the changes in crystallinity which also increases rapidly in the beginning, levelling off at higher doses. Higher spatial concentrations of free radicals and their efficient recombination at higher doses can be inferred from this observation (1). This also implies that the crystallinity increase observed in bulk UHMWPE samples irradiated in air is due to chain scission and that the scission is caused by oxidative degradation of the polymer chains upon irradiation. However, for the irradiations conducted in nitrogen atmosphere, the polymer did show an increase in crystallinity (Fig.9)* But this increase is less (6 cal/gm) as compared to that for the samples irradiated in air (10 cal/gm). Besides, very small concentrations of carbonyl groups were detected in the former samples (Fig.5). 35 From the fact that samples irradiated in nitrogen atmosphere also exibited an unprecedented crystallinity increase it can be deduced that some amount of chain scission in bulk UHMWPE may occur even in the absence of oxygen. An appreciable degree of chain rupture in vacuo has been postulated in the past to explain the unattainibility of 100^ gel in polyethylene even at high radiation doses. Generally the ratio of chain scission to crosslinking, for HDPE, has been reported to be 0.2 (9). Besides, Pennings (25) had found a rapid decrease in the tensile modulus of high molecular weight polyethylene fibers of molecular weight 4x106 upon irradiation in vacuum. This decrease was highly non-linear at low doses upto 5 Mrad. From this it was concluded that chains under stress (like the tie molecules) are preferential sites for scission. For the first time the ratio of chain scission to crosslinking was speculated to be as high as 1.0 for polyethylene. Surprisingly majority of the crystallinity changes take place within the first 5 Mrad dose (Fig.3). This implies that the processes that are responsible for the crystallinity changes occur as soon as the polymer is exposed to ionizing radiation. A significantly higher preference for the reactions leading to chain scission is suggested. The fact that these changes are enhanced by the presence of oxygen is also clear from the magnitudes of the changes in crystallinity and carbonyl content 36- in bulk UHMWPE samples irradiated in air. These observations thus reinforce the thinking that the tie molecules are the primary points of attack by high energy radiations. Besides, this dose is in the range that is commercially used for medical sterilization of plastic implants. This, at least partially, justifies why the hip cup made from UHMWPE and gamma sterilized before implantation showed a brittle fracture and high degree of crystallinity after implantation in the human body (26). The crystallinity changes in UHMWPE powder are quite interesting. These samples are similar to HDPE owing to their comparable crystallinities. Generally, HDPE shows no crystallinity changes upon irradiation, atleast at low doses. But its crystallinity upon recrystallization drops rapidly with the radiation dose (Fig.13,14) evidently due to the incorporation of crosslinks in the polymer. These crosslinks have been shown to be limited to the disordered region (4). We found that the crystallinity of UHMWPE powder upon irradiation decreases under nitrogen atmosphere (Fig.12) from 19-2% to 11% while it increases under oxygen rich atmosphere (Fig.11) from 19-2% to 83%. These changes are relatively small but significant compared to those in bulk UHMWPE. UHMWPE powder is almost 79% crystalline and because of its high melting temperatures it is also speculated to have an extended chain morphology. Incorporation of crosslinks or double 37 bonds in the extended chain crystals would destroy the order in its vicinity. This is what probably happens when UHWMPE powder is irradiated in nitrogen atmosphere. Oxygen, on the other hand, would facilitate chain scission. The increase in crystallinity that we found could be attributed to the scission, followed by recrystallization, of certain chains in the amorphous phase or incorporation of the severed chains in the crystal lattice. In fact, large amounts of carbonyl groups (G ~ 25) were detected in this sample (Fig.5). In addition, it was observed that the UHWMPE powder irradiated in oxygen not only dissolves readily in hot xylene but, like HDPE, also forms white crystals on cooling the solution to room temperature. Unirradiated UHMWPE, on the other hand, gels on cooling the solution to room temperature. It is surprising, however, that this crystallinity effect should persist even at such high levels of crystallinity. It must be noted that this effect was not observed in HDPE. Thus these observations provide an important clue to the morphological subtleties of these polymers having same chemical structure and degree of crystallinity but different molecular weights. Crystallinity effect in UHMWPE with extended chain morphology has also been reported by Bhateja (5). It was shown that the crystallinity of UHMWPE can be increased from 67.4% to almost 80% by pressure crystallization followed by irradiation. Surprisingly, appreciable amounts of carbonyl were detected 38 in the UHMWPE powder irradiated in nitrogen atmosphere and exposed to air for about one month subsequent to irradiation. However, when the powder was melted in the sealed tubes before exposure to air, very little amounts of carbonyl were found (Fig.5). UHMWPE powder is almost 79% crystalline and hence we suspect that some of the free radicals may remain trapped in the crystalline regions. On exposure to air, these free radicals can readily react with the atmospheric oxygen during the post-irradiation period and eventually yield carbonyl groups. For low irradiation doses, the growth of trans-vinylene unsaturation is reported to be linear with the dose (10). The formation of this species is reported to be independent of temperature, from the glass temperatures to the temperature of the pure melt. The yields of trans-vinylene were also reported to be independent of the level of crystallinity. We observed a linear increase of trans-vinylene unsaturation with the radiation dose upto 20 Mrad dose (Fig.6). The yields of trans-vinylene in this figure are given relative to the concentration of methyl groups in the material, which are reported to remain unaffected upon irradiation. Relatively higher yields of trans-vinylene unsaturation in irradiated UHMWPE powder are, however, obtained (G ~ 0.8). The formation of trans-vinylene was also found to be uninfluenced by the irradiation environment. Similar trans-vinylene yields (G ~ 0.64) were obtained in the bulk UHMWPE 39 samples. The yields in the latter were only 80$ of those in UHMWPE powder. A distinct influence of the morphology of the polymer on trans-vinylene formation is suspected. In fact, Patel (27) has reported that the formation of this species preferentially occurs in the crystalline core of the polymer. In this respect it is interesting to note that this sample is about 79$ crystalline as compared to 50$ crystallinity of the bulk UHMWPE. The G values observed for trans-vinylene formation in irradiated UHMWPE were considerably lower than are reported for linear polyethylene (1.5-2.4) (27,28). We also observed that the radiation yield of trans-vinylene in UHMWPE powder is similar to that in HDPE. It must be noted that very small dose rate was used for irradiation of the polymer and we suspect this to be responsible for the lower trans-vinylene yields. The peak melting temperatures also showed increases of variable degrees in all the bulk UHMWPE samples (Fig.4). Scission of the entangled chains may result in relaxation of some microstresses in the crystallites, thereby reducing the number of defects and increasing the melting temperature. On the contrary, a monotonous decrease in peak melting temperature was observed in UHMWPE powder irradiated in both nitrogen and oxygen atmosphere. UHMWPE is speculated to have an extended chain morphology based on its high melting point. Formation of crosslinks, double bonds MO or carbonyl groups in the crystal lattice would incorporate defects in the lattice thereby causing the melting point to decrease. This can be inferred from the rapid decrease in melting temperature and the corresponding higher yields of carbonyl groups in UHMWPE irradiated in oxygen. In addition, relatively higher yields of trans-vinylene (G ~ 0.8) are detected in UHMWPE irradiated in nitrogen atmosphere. Inclusion of crosslinks in this sample can be inferred from the continuous decrease in its crystallinity on second melting with the radiation dose (Fig.12). The similarity of the crystallinity changes in slowly (SC) and quench (QC) crystallized samples is very striking (Fig.3)« The initial difference in the heats of fusion of these two samples before irradiation is about 6 cal/gm. Upon irradiation both the samples show identical changes in the heat of fusion. Quench crystallization of UHMWPE would be expected to produce imperfect crystallites and also a large amount of amorphous region with substantial microstresses in it. The chains in this region would be relatively more vulnerable to radiation attack. The same amounts of increases in the heats of fusion at each irradiation dose would imply similar degrees of chain ruptures in both the samples. It can be imagined that a given radiation dose may initiate a maximum number of reactions and that this maximum may already have been attained in the slowly crystallized samples 41 which have almost 50% amorphous material. Increasing the amorphous content any further may not initiate more number of reactions. This explanation, however, needs further experimental confirmation. The crystallinity of HDPE (76%) is found to remain unaffected for all the radiation doses used (Fig.13)- A slight increase was reported by Bhateja (14). High degree of crystallinity of HDPE manifesting into low number of tie molecules (which are presumed to be responsible for the crystallinity effect) may not cause any crystallinity changes at all. At such low doses of radiation the crosslinking effect would predominate. The incorporation of crosslinks in the material is clearly demonstrated by the rapid decrease in crytallinity (from 76% to 62%) of irradiated HDPE samples upon recrystallization in the DSC at a cooling rate of 20 C/min (Fig.13,14). This decrease is almost twice of that found in UHMWPE. However it must be realized that owing to the large number of entanglements in UHMWPE the effect of crosslinks may not be reflected in the crystallinity changes on second melting to a substantial extent. In fact the changes in crystallinity on first and second melting indicate that the role of chain scission is more consequential in the radiation effects of UHMWPE than its counterpart, the crosslinking. 42 CHAPTER V i CONCLUSIONS UHMWPE, varying in crystallinity from 40 - 80^ and irradiation environment (inert and oxygen rich), was irradiated upto 20 Mrad. The irradiation effects were characterized in terms of the heat of fusion (or crystallinity), melting temperatures and the chemical changes detected by infrared spectroscopy. These j studies have revealed some of the very unusual irradiation ! j effects in Ultra-high Molecular Weight Polyethylene which are as I 1 follows: ( (i) It is now unequivocally established that the increase in crystallinity of UHMWPE upon irradiation takes place, atleast to an appreciable amount, irrespective of the environment during irradiation. Chain scission, even in the absence of oxygen, is not totally Impossible. Reactions involving ionic intermediates are known to cause molecular fragmentation too. Besides, the tie molecules, which are probably under stress, are the weakest part of the chains and would be severed preferentially. Even though a direct proof has not been presented to assert the reaction paths from the inception to the ultimate fate of the free radical formed, it can be deduced from the experimental evidences that 43 chain scission is, for the first time in this polymer, a more significant effect over its counterpart, the crosslinking. (ii) The rapid changes in the first 5 Mrad dose imply that whatever processes that are occuring are relatively more probable processes. The recombination reaction of the free radicals formed by C-C bond scission is prevented by the "snapping” apart of the severed chains. The inherently high crystallization rates of linear polyethylene and the large amorphous fraction in bulk UHMWPE provide sufficient driving force for the process to occur. The scission of C-C backbone is further assisted by the availability of dissolved oxygen in the polymer. In essence, this observation implies that the chains in bulk UHMWPE are under a stress and they show a higher tendency to relax upon deposition of high energy photons, through the rupture of C-C backbone in the amorphous regions. (iii) A steady increase in peak melting temperatures with increasing radiation dose is encountered in all the bulk UHMWPE samples. In the absence of lamellar thickening (14) it can be deduced that this is a consequence of some molecular relaxations and a reduction in the number of defects in the crystalline regions. This "annealing" effect could only be a manifestation of the scission of the stressed chains in the amorphous phase or the tie molecules connecting different crystallites. 44 (iv) A striking decrease in the melting points of the UHMWPE powder irradiated in both nitrogen and oxygen atmosphere provides an important clue to the morphological differences in the bulk and powder UHMWPE. It has been speculated that UHMWPE powder has an extended chain morphology (17). The decrease in crystallinity when irradiated in nitrogen atmosphere and the increase on irradiation in oxygen atmosphere also suggest the extended chain morphology. Incorporation of crosslinks or double bonds in the polymer in former samples would destroy the crystallinity in its vicinity. Oxidative degradation would, on the contrary, facilitate further crystallization. But both, the crosslinks as well as the carbonyl groups (resulting from oxidation), are defects in the crystal lattice and hence can cause a decrease in the crystal perfection (or melting temperature) of irradiated UHMWPE powder. The tremendous difference between the melting temperatures of UHMWPE powder and bulk UHMWPE (~147°C against ~137°C) also highlights the morphological differences of the two samples. (v) Quenching of UHMWPE from melt did not induce any additional crystallinity changes in comparison to the slowly crystallized samples. This is quite unexpected because quenching of UHMWPE melt is found to reduce its crystallinity to as low as 42%. Quenching is also known to produce imperfect crystals and considerable amount of stresses in the polymer chains. Scission 45 of these chains should have occured more readily and to a greater degree in quenched samples. But this is not found. It can be inferred that the crystallinity of slowly crystallized UHMWPE is in itself so low (48%) that reducing it any further may not stimulate additional degradation of the polymer upon irradiation. Further experimental evidences are, however, required to confirm the above explanation. (vi) The formation trans-vinylene is found to be independent of the environment during irradiation. It is already known to be independent of the level of crystallinity and the temperature of irradiation. Its linear increase with the radiation dose for all samples is in harmony with the observations reported in other polyethylenes. However, relatively higher yields of trans-vinylene in UHMWPE powder irradiated in nitrogen atmosphere indicate a strong morphological dependence. Patel (27) has reported a preference for the formation of this species in the crystalline regions. UHMWPE powder, indeed, has a higher degree of crystallinity (79%). Yet, a confirmation of this observation is warranted. (vii) The G values for the formation of trans-vinylene in our studies are quite low (0.64-0.8) as compared to those reported in the literature for linear polyethylene (1.5-2.4). Realizing the difference in the dose rates used by us (0.15 Mrad/hr) against those used by others (3 Mrad or more) we suspect that this observation is a manifestation of the rate of delivering the radiation dose to the polymer. Because of the large magnitude of the differences it is important to testify the trans-vinylene yields by some more accurate quantitative methods (e.g. chemical method such as bromination of double bonds in the polymer). (viii) The amounts of carbonyl formed in each sample tend to reach a saturation at higher radiation doses. Crosslinking and oxidative chain scission are competitive processes at all radiation doses. At low radiation doses the scarcity of free radicals formed favors the latter. Large amounts of free radicals at higher doses may facilitate efficient radical recombination thereby arresting further oxidation. Reduction in the crystallinity increase of bulk UHWMPE at higher radiation dose reported by Bhateja (14) can be clearly attributed to the increase in the efficiency of free radical recombination at higher radiation doses. The peak in the carbonyl yield of bulk UHMWPE samples at 2.5 Mrad dose may be a consequence of the sparcely formed free radicals, making the recombination process almost unlikely. The role of dissolved oxygen is of tremendous significance in this respect in reacting with the free radicals and thereby yielding relatively higher concentrations of carbonyl groups at 2.5 Mrad dose. 47 REFERENCES 1) Ungar G.; "Radiation effects in Polyethylene and n-alkanes"; J. Mat. Sci., 16; 2635(1981). 2) Bassett D.C.;"On Moire patterns in electron microscopy of polymer crystals"; Phil. Mag., 10, 595(1964). 3) Mandelkern L.,Chapter 13, page 287;in "The radiation chemistry of macromolecules"; Vol. I, Edited by M. Dole (Academic Press,New York,1972). 4) Patel and Keller; "Crystallinity and the effect of ionizing radiation in polyethylene. I. Crosslinking and the crystal core"; J. Polym. Sci. Polym. Phys. Ed., 13» 303(1975). 5) Bhateja S. K.; "Radiation-induced crystallinity changes in pressure-crystallized UHMWPE"; J. Macromol. Sci., B22(1), 159(1983). 6) Dole M. (Ed.); "The radiation chemistry of macromolecules"; Vol. I and II, Academic Press, New York, 1972. 7) Keller A. and Ungar G.; "Radiation effects and crystallinity in polyethylene"; Radiation Phys. Chem., 22, 155(1983). 8) Grubb D.T.; "Radiation damage and electron microscopy of organic polymers"; J. Mat. Sci., 9, 1715(1974). 9) Lyons B.J.; "The effect of radiation on the solubility and other properties of High and Linear low density polyethylene"; Radiation Phys. Chem., 22, 135(1983). 10) Mandelkern L., Chapter 13, page 303;"The radiation chemistry of macromolecules"; Vol. I, Edited by M. Dole (Academic Press, New York, 1972). 11) Keller and Patel; "Crystallinity and the effect of ionizing radiations in PE. 1.Crystallinity and the crystal core"; J. Polym. Sci. Poly. Phys. Ed., 13, 303(1975). 12) Ungar G., Grubb D.T. and Keller A.; "Effect of radiation on the crystals of Polyethylene and paraffins: 3* Irradiation in the electron microscope"; Polymer, 21, 1284(1980). 13) Roe et al; "Effect of radiation sterilization and aging on Ultrahigh Molecular Weight Polyethylene";J. Biomed. Mat. Res., 15, 209(1981). 14) Bhateja S.K. and Young R.J.; "Radiation-induced crystallinity changes in polyethylene"; J. Polym. Sci. Polym. Phys. Ed., 21, 523(1983). 49 15) Bhateja S.K.; "Radiation-induced crystallinity changes in Linear Polyethylene"; J. Appld. Polym. Sci., 28, 861(1983). 16) Bhateja S.K. and Andrews E.H.; "Effect of high energy radiation on the uniaxial tensile creep behaviour of Ultrahigh Molecular Weight Polyethylene"; Polymer, 24, 160(1983). 17) Zachariades A.E. and Logan J.A.; "The melt anisotropy of Ultrahigh Molecular Weight Polyethylene"; J. Polym. Sci. Polym. Phys. Ed., 22, 821(1983). 18) Zapas L.J. and Crissman J.M.; "Creep and recovery behaviour of UHMWPE in the region of small uniaxial deformations"; Polymer, 25(1), 57(1984). 19) Provided by H. McKellop, Biomechanics Research Lab., Orthopaedic Hospital/University of Southern California. 20) McLaughlin W.L.;"Radiation Processing Dosimetry"; Radiation Phys. Chem., 21(4), 359(1983). 21) Luongo J.P. and Salovey R.; "Infra-red spectra of Irradiated Polyethylene"; J. Appld. Polym. Sci., 7, 2307(1963). 22) Wunderlich B. and Cormier C.; "Heat of fusion of polyehtylene"; J.Polym. Sci. A-2, 5, 987(1967). 23) Iring M., Tudos F. et al; "The thermo-oxidative degradation r 50 of polyolefins. Part 10: Correlation between the formation of carbonyl groups and scission in the oxidation of PE in the melt phase”; Polymer Degradation and Stability, 2, 142(1980). 24) Benham J.V. and Pullukat T.J.; "Analysis of the types and amounts of carbonyl species present in oxidised polyethylene”; J. Appld. Polym. Sci., 20, 3295(1976). 25) de Boer J. and Pennings A.J.; "Crosslinking of Ultrahigh strength polyethylene fibers by means of gamma-radiation”; Polymer Bulletin, 5, 317(1981). 26) Grood E.S., Shastri R. and Hopson C.N.; "Analysis of retrieved implants: Crystallinity changes in Ultrahigh Molecular Weight Polyethylene”; J. Biomed. Mat. Res., 16, 399(1982). 27) Patel G.N.; "Crystallinity and the effect of ionizing radiation in polyethylene. V. Distribution of trans-vinylene and trans,trans conjugated double bonds in linear polyethylene"; J. Polym. Sci. Polym. Phys. Ed.-, 13, 351(1975). 28) Ibid, page 322. i 51 APPENDIX TABLE 2 HE-1900 [CM,SC] + + — Rad. + . + 1st Melting Ilnd Melting dose 4. -------4------ I I I Tp I Tm I I 1 Heat 1 of 1 fusion I I I I % I Tp I Tm I Cryst.I I I Heat I I of I I fusion I % Cryst Mrad I °C I °C Ical/gm I | °C I °c I cal/gml 0.0 I I I 137.0 I 1*15• 2 1 1 33.19 | I I I I 48.1 I 139.3 I 155.3 I I I I I I 34.36 I | | 49.8 2.5 1 1*10.4 I 1*18.0. 1 I 39.64 I 57.4 I 134.7 I 144.1 I I I I 36.05 I | | 52.2 5.0 1 143.1 I 152.3 1 1 I 40.82 I 59.2 I 13*1.0 I 146.0 I I I I 35.69 I | | 51.7 10.0 1 145.2 I 155.4 I I 41.58 j I 6O.3 I 130.5 I 1*19.1 I I I I 33.79 I I | 48.97 20.0 1 145.5 1 156.1 1 1 I 43.66 I 63.3 I 128.8 I 147.2 I I I j 33*63 j 48.7 -+ 4. -------4-------4 .----- ---- 4-- _ " I - -------- + ----------- - f Ln NJ TABLE 3 HE-1900 [CM,QC] Rad. dose + 1st Melting Tp I I Heat I I I of I % I Tp I fusion I Cryst.I — +. Tm Ilnd Melting I Heat I Tm I of I % I fusion I Cryst. Mrad °C c 1 -------- cal/gm v— ---- t °c °c 1 - -------H ! cal/gm 0.0 136.7 ■ 149-1 28.87 41 .80 139.3 155.3 34.36 49.80 2.5 138.1 . 147.8 35.33 51 .20 135.5 148.6 34.81 50.40 5.0 138.3 148.1 35.13 50• 90 134.2 146.7 35.21 51.00 10.0 138.5 148.1 37.27 54.00 132.9 143.8 32.90 47.70 20.0 141.8 151.3 38.70 56.10 129.4 146.0 31.59 45.80 TABLE 4 HE-1900 [NITROGEN] I Isb Melting I Ilnd Melting I Rad. I I I dbse +-------h-------+------- - t ------- 1 - -------+-------+-------+-------+ I I I Tp I I I Tm I Heat I I of I Ifusion I I % I Cryst.I I Tp I I Tm I Heat I I of I Ifusion I % Cryst Mrad I °C I °C I cal/gm J °C I °C I cal/gm| • 0.0 I 1H0.1 I I | 154.1 ! 33-95 I 49.20 I | I 139.3 I I 155.3 34.36 I | | 49.80 2.5 I I I 142.3 I I I 154.9 I 37.38 I I | 54.20 I I 131-8 I I 144.3 I 37.10 I | | 53-80 5.0 I 145.1 I I | 157.8 I 38.39 I I 55.60 I 131-9 I I 147.4 I 36.75 I 53.30 10.0 I 144.8 I i 156.2 I 39.46 I 57.20 I I 130.8 I 1 143.7 I 34.56 I | | 50.10 20.0 I 146.9 I I I 159-9 I 40.07 I I I I 58.10 I I 131.8 I I 148.7 I 31.78 I I I 46.10 TABLE 5 HE-1900 [VACCUUM] I 1st Melting I Ilnd Melting Fad. I I dose +------- I I Tp I Tm I Heat I of (fusion I % I Cryst.I I Tp I I Tm I Heat I I of I % Ifusion I Cryst. Mrad I °c °C Ical/gm I °C I °C I cal/gmI 0.0 I I m o .2 153-2 I I 34.80 j 50.40 I j I 139.6 I | 156.3 I I I 34.08 I 49.40 | | 2.5 | l 143.1 156.0 I 37.60 1 54.50 I | 135.0 1 | 148.2 I 34.33 I 49.80 | | 5.0 I ! 144.0 ■155.0 1 1 38.10 55.20 I | 133.9 I 146.3 I 32.70 I 47.40 I | 10.0 I I 145.2 155-9 I 39.71 57.60 I | I I 1 1 1 | | 20.0 | I 146.8 I 158.7 I 40.45 I 58.60 I I 136.6 | I 148.5 1 30.93 1 44.80 1 1 -t- I I I ■+ I I I + I + TABLE 6 EXTRUDED UHM Pad. dose +- 1st Melting Ilnd Melting Tp Tm +. + Heat I I of I % I Tp Ifusion I Cryst.I + --------------------- +.------------- + C I cal/gm I +-------- +---- I fusion Cryst. cal/gm Mrad 0.0 I 138.U | 145.0 34.89 50.57 135.4 143.2 31-99 46.40 2.5 I 143.2 156.0 40.10 58.10 136.5 150.5 32.73 47.40 5.0 I I 144.2 154.2 41.92 60.80 135.2 149.3 32.65 47-30 10.0 I 147.2 158.0 42.07 61.00 141.1 153-5 31.21 45.20 20.0 I 146.8 157.3 41.96 60.80 139.3 150.3 30.66 44.40 Rad. TABLE 7 HE-1900 [OXYGEN,PJ +------------- 1st Melting Ilnd Melting dose +-------+- I I I Tp | Tm I Heat I of I fusion I I I I % I Tp I I Cryst.I I Tm I Heat I I of I Ifusion | % Cryst. Mrad I °c I °C Ical/gm I I °C I °C I cal/gmI 0.0 I I I 146.8 I 156.0 I I 54.64 I 79.2 I 130.7 I I I 140.3 I I I 38.95 I | | 56.4 2.5 I j I 142.3 i i | 150.3 I 55-18 1 I I I 79-97 I 130.3 I I I I 141.0 I 39.01 I j | 56.5 5.0 1 143.1 I 151.6 I I 55.76 I I 80.8 I 133.1 I I I I 142.2 I 40.82 | j | 59.2 10.0 I | ! 141.6 I 150.0 I 57.35 I 83.12 I 132.4 I I I l 141.1 I 41.24 I | | 59.8 20.0 I 141.8 I 149.7 I 57.23 i i i I 82.94 I 130.8 I 140.3 I 40.17 I 58.2 + + + +---- — --+ + + + Ln •sj TABLE 8 HE-1900 [NITRO,P] Rad. I I I 1st Melting I I I Ilnd Melting dOoS — — — — I I I Tp I Tm I Heat I of I fusion I % I Cryst.I I Tp I I Tm I Heat I I of I Ifusion I % Cryst. Mrad I °c I °C Ical/gm i °C I °C I cal/gmI 0.0 I I I 1*16.8 1 1 | 156.0 I I 54.64 i I 79.2 I i I 130.7 I | 140.3 I I I 38.95 I | | 56.4 2.5 I 1*14.6 1 152.6 I 54.71 79.3 I I 129.7 I | 139.7 I 38.71 I I | 56.1 5-0 1 144.3 ! I | 151.6 I 53-18 I 77.1 | I 125.8 I i 136.0 I 37.67 I I | 54.6 10.0 I 143.8 I j j 152.2 I 54.20 78.6 I i 126.1 I 135.7 I 37.78 I | ) 54.8 20.0 I 143.2 ! I i 151.0 I 52.86 I 76.6 I I 125.4 ! I 134.1 I 36.67 I I I 53.1 TABLE 9 HOPE [SC] Rad. dose I I Isb Melting 1 1 1 Ilnd Melting I I I ! TP ! i i Tm I Heat I of I fusion 1 1 1 % 1 1 Cryst.I 1 Tp 1 1 Tm I Heat I I of I Ifusion I I % I Cryst.I Mrad I °C I °C Ical/gm °C 1 °C I cal/gmI I 0.0 I 137.7 ! 1*17.5 I 52.62 1 1 1 76.3 1 1 1 ' 1 135.1 I I 1*15.0 I I I 50.50 I I I I 73-20 I I 2.5 I 139.3 I 1*19.8 I 52.86 1 76.60 I | I 137.4 I I 148.3 1 47.87 1 1 | 69.4 I I 5.0 I 139.8 I I i 150.0 I 52.*10 I 1 75-90 1 1 i 137.7 I 1 147.9 1 45.81 ! i i 66.40 I I 10.0 I I I I i 1 1 i i 1 1 i 1 1 1 i 1 1 1 | I i 20.0 I | j 139.6 I 150.0 1 1 52.85 1 1 76.60 1 1 1 131.*1 1 1 142.5 1 41.59 I I I I 60.30 I I TABLE 10 HDPE [QC] Rad. dose + 1st Melting I Tp Tm I Heat I of I fusion % Cryst. I Tp I I Tm I Heat I I I of I % i Ifusion I Cryst.I Mrad I °c °C I cal/gm °C ! °C I cal/gmI I 0.0 I 135.2 149.1 I I 49.78 72.1 i 136.4 I I 147.2 I I I I 50.66 I 73*4 1 1 1 1 2.5 I 133-0 141.0 j I 47.86 69.4 133-1 I i 140.8 I 46.75 1 67.8 1 1 1 1 5.0 I 137.1 148.8 | I 48.58 70.4 135.3 I I 148.5 I 46.74 1 67.7 1 1 1 1 10.0 I 134.6 142.8 I 49.17 71-3 133.8 I I 143.2 1 45.74 1 66.3 ! 1 1 1 20.0 I 134.9 147.2 I I 48.75 I 70.5 132.4 I I 142.8 1 '42.09 I 61.0 I ! I I Ilnd Melting

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Creator Shinde, Ashok N. (author)

Core Title Irradiation effects in Ultrahigh Molecular Weight Polyethylene

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Degree Program Chemical Engineering

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Advisor Salovey, Ronald (committee chair), Chang, Wenji Victor (committee member), Shing, Katherine S. (committee member)

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