Studies on the Crystallization of Poly(Vinylidene Chloride-Co-Vinyl Chloride)

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Polymer Journal, Vol.33, No. 11, pp 835—841 (2001)

Studies on the Crystallization of Poly(vinylidene chloride-co-vinyl chloride)

† Chao-Lin CHEN, Tar-Hwa HSIEH, and Ko-Shan HO

Department of Chemical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-Kuo Rd., San-Ming District, Kaohsiung 807, Taiwan

(Received November 8, 2000; Accepted August 13, 2001)

ABSTRACT: The degradation of Poly(vinylidene chloride-co-vinyl chloride) (VDC-VC copolymer) is found by the less order X-Ray diffraction patterns of annealed samples at 175◦C. The degraded VDC-VC copolymer releases 1,3,5- trichlorobenzene, which behave as the nucleus of recrystallization during cooling and increase the crystallinity. The recrystallization behavior demonstrates that it is very sensitive to the degradation of VDC-VC copolymer. The degraded VDC-VC copolymer has an enhanced nucleation effect at lower temperature (90◦C) resulting in higher crystallinity and rate of recrystallization regardless of better diffusion ability at higher temperatures. The polarized optical pictures reveal the growing of spherulites and the average size is about 2.9 mm. KEY WORDS Poly(vinylidene chloride-co-vinyl chloride) (VDC-VC copolymer) / Recrystalliza- tion / Degradation / Poly(vinylidene chloride-co-vinyl chloride) (VDC- was increased to 100–150◦C, no significant change was VC copolymer) was proved to own high crystallinity, found. This suggests that the presence of plasticizer high density and excellent gas barrier properties. Af- cannot solve all the problems due to the low growing ter suitable thermal treatment and orientation along the rate of crystallization and the thermal degradation of uni-direction, a high performance plastic film can be VDC-VC copolymer. Burnett et al.6 pointed out that obtained, which is used as an important packing mate- crosslinking, decolorination, and change of crystalline rial in food industry.1–4 morphology would happen when the conversion of de- Even though VDC-VC copolymer has been widely hydrochlorination of VDC-VC copolymer is close to used as food-packing materials, few literature men- 1%. Therefore, in addition to plasticizer, some stabi- tioning about its crystallization, procession, and ther- lizer should be added in order to remove the unfavor- mal treatment behaviors are available due to the rather able side effects of thermal degradation in the studies complicated crystallization behaviors, which are influ- of crystallization kinetics or morphology of VDC-VC enced by composition, mechanical/thermal treatment, copolymer. It was shown that9, 10 the degradation dur- and degradation.4, 5 Some people claimed the crys- ing the thermal treatment will influence the molecular tallinity and the rate of crystallization of VDC-VC structure, crystallization behavior, and morphology of copolymer decreases with the vinyl chloride monomer the final VDC-VC copolymer product. Consequently, (VCM) content and the induction time of crystallization it is necessary to understand its influence on the melt- increase with VCM composition.4–6 If recrystalliza- ing and crystallization of copolymer in this study. Jonas tion occurs at temperature below 100◦C when quenched et al.10 also indicated the relationship with crystallinity, from melt state, the very tiny crystals with lower de- nucleus density, and impingement of spherulite except gree of crystallization makes the studies on the mor- thermal treatment conditions. phology and kinetics of crystallization more difficult. A commercially available VDC-VC copolymer was When recrystallization temperature was above 100◦C, adopted to avoid the possible degradation during crys- the crystal size is larger, however, the studies can- tallization or melting. The effect of melting temper- not be performed due to the occurrence of thermal ature and melting time on the crystalline behavior or degradation.7, 8 Reinhardt7 tried to decrease the crys- morphology of VDC-VC copolymer can be analyzed tallization induction time by adding slight amount of by isothermal differential scanning calorimetry (DSC), plasticizer. He found that the glass transition tem- and polarized optical microscopy (POM). Based on the perature (Tg) and crystallization temperature decreased obtained data, an isothermal crystallization kinetics and due to the increase of molecular mobility and induc- possible crystal growth mechanism can be set up, which tion time of recrystallization is hence reduced. These can then be applied to predict the microstructure before data were measured at low temperature. If temperature and help us to adopt the suitable procession technology.

†To whom correspondence should be addressed (Fax: +886-7-3830674, E-mail: [email protected]).

835 C.-L. CHEN, T.-H. HSIEH, and K.-S. HO

Optical Microscopy (POM) EXPERIMENTAL An Olympus polarizing microscopy with a Linkam THMS-600 heating/freezing stage was used to observe Material the morphological change. VDC-VC copolymer films The VDC-VC copolymer, commercial-grade film, were cut and put in the hot stage between two thin glass which contains ca. 2 wt% of acetyl tributyl cirate and slides. The hot stage was first kept at ambient tem- epoxidized soy bean oil behaving as plasticizer and perature for 20 min, which was considered to be long chemical stabilizer, respectively, was obtained from enough to purge the system with nitrogen. The heating ◦ −1 Dow chemical Co., Ltd., USA. rate is 100 C min and experiments were conducted at Thold for some thold under nitrogen, and then cooled to − ◦ −1 Characterization recrystallized temperature (Tc)at 100 C min . Pic- The VDC-VC copolymer was first purified with tures were taken at each isothermal crystallization and tetrahydrofuran (THF), and then adds some of ethanol, melting by a Nikon auto microflex camera. the white powders of VDC-VC copolymer were sepa- rated by filtration. The obtained VDC-VC copolymer X-Ray Diffraction Patterns powder was characterized by elementary analysis (EA) The X-Ray diffraction patterns of samples were ob- using Heraeus CHN-rapid and Tacussel Coulomax 78. tained from exposure to a Siemen D5000 X-Ray source The results are shown in Table I. According to EA re- with Cu-Kα (1.542A) as target. The diffraction angles sults and after calculation, we know the mole percent- (2θ) ranged from 2 to 40 degree with a power of 40 KV age of VDC comonomer in VDC-VC copolymers is and 30 mA. about 83%. RESULTS AND DISSCUSSION Differential Scanning Calorimetry (DSC) Analysis Melting and Non-isothermal Crystallization Studies. The Effect of Melting Conditions on the Crystallization The melting and non-isothermal crystallization behav- Behavior ior of VDC-VC copolymer were conducted in a Perkin– Figure 1 includes series of heating and cooling cycles Elmer DSC-7 calibrated with indium, and operated un- of VDC-VC copolymer, which obtained from ramping der a constant gas flux. Samples approximately 4–6 mg DSC scans. The main melting peak comes from the in weight were first scanned at 20◦C min−1 from am- regular chain-folded types of crystallization, which has bient temperature to a chosen temperature (Thold) and a more regular structure and demonstrates higher melt- held at this temperature in the molten state for a cho- ing temperature. ◦ −1 sen time (thold) and then quenched at 20 C min back From the previous studies on the pure poly(vinyli- to ambient temperature. All samples were reheated and dene chloride) (PVDC), an early stage of degrada- then recooled under the mentioned conditions for three tion is observed at temperature slightly higher than 11–13 times. The melting and nonisothermal crystallization the melting point. The degradation behavior in- characteristics of samples can then be observed. Isothermal Crystallization Studies. Isothermal crystallization was also performed in nitrogen in the same way by Perkin–Elmer DSC-7. Samples were first heated in the calorimeter with 20◦C min−1 from ambient temperature to a Thold and held at this temper- ◦ −1 ature for a thold and then quenched at 320 C min to a chosen recrystallization temperature (Tc), and held at this temperature for different time. The Thold values ◦ varied from 170 to 180 C, while thold ranges from 2 to 7 min. For each experiment, crystallization kinetics was evaluated by Avrami’s method.

Figure 1. Typical melt crystallization plots of VDC-VC Table I. Elementary analysis of VDC-VC copolymer ◦ copolymer with a melt holding temperature at 175 C and a hold- CHCl ing time of 2 min. (a), (b), and (c) represent the sequence of ramp- (CH2CCl2)x(CH2CHCl)1−x Calc. 26.32 2.42 71.26 ing temperature runs. (a ), (b ), and (c ) represent the sequence of Found 26.34 2.45 71.21 cooling temperature runs.

836 Polym. J., Vol.33, No. 11, 2001 Studies on the Crystallization of Poly(vinylidene chloride-co-vinyl chloride)

Scheme 1. Differrent types of degradation PVDC volves the evolution of hydrochloric gas along the main T chains resulting in the conjugation alternating double hold＝170℃ T ＝175℃ and single bonds along the main chains, which does hold ) 11 c Thold＝180℃

not induce any scission effect of backbones. Another X types of degradation following the dehydrochlorination is the unzipping reaction of the conjugated backbones which produce 1,3,5-trichlorobenzene or intermolecu- lar crosslinking between the neighboring conjugated Crystallinity ( backbones.11 Similar reaction as shown in Scheme 1 is found in the VDC-VC copolymers and the crystallization behavior of VDC-VC copolymers will be in- fluenced by the degradation effect. Since 83% of VDC is present in the VDC-VC copolymers, the crystallization behavior will be sim- Figure 2. Crystallinity of VDC-VC copolymer at various melting temperatures with different sequences of scans. ilar to pure PVDC. In the non-isothermal crystallization studies, de-nucleation temperature (Thold), de- nucleation time (thold), and sequence of re-heating scans VDC-VC copolymers may result in a by-product 1,3,5- (N) are three main factors which can influence the crys- trichlorobenzene except hydrochloric gas. Due to the tallinity, heat of recrystallization (Hc), and super- high boiling point of 1,3,5-trichlorobenzene (204.8◦C), cool behaviors of VDC-VC copolymer. The degree of it will not be evaporated at holding temperatures (170, ◦ crystallinity (Xc) is calculated by dividing the heat of 175, or 180 C) and will act as the nucleus for re- fusion from the DSC thermograms by the theoretical crystallization during cooling, which would then in- 7 heat of fusion of perfect unit cell. And supercooling crease the crystallinity. For higher Thold, the numbers phenomenon is represented by the difference between of nuclei made of condensed 1,3,5-trichlorobenzene is melting point (Tm) and the subsequent recrystallization more than those at lower ones and the Xc is increased temperature (Tc) and designated here as T. also. Increasing numbers of scanning will result in an Regularly, Xc is decreased if the recrystallization is imperfect structure of the crystals and the decreasing of started from a higher Thold. However, Figure 2 which is the Xc at each Thold as also shown in Figure 2. a diagram of Xc vs. sequence of re-heating numbers at The influences of numbers of scanning and Thold on ◦ various Thold (170, 175, and 180 C) demonstrates dif- supercooling behavior (T) are demonstrated in Fig- ferent types of crystallization behavior. Crystallinity ure 3. The supercooling effect decreases when numbers of VDC-VC copolymer increases at higher Thold due of scanning increase because higher N values mean to the degradation effect, which is already present longer holding time, which can shorten the difference 11–13 at the initial stage of Thold. The degradation of between Tm and Tc. Again, the effect of holding tem- Polym. J., Vol.33, No. 11, 2001 837 C.-L. CHEN, T.-H. HSIEH, and K.-S. HO ) 1 － C) 0 ( T

(J mol c △ H

T △

0 Thold ( C)

Figure 3. Supercooling effect (T) of VDC-VC copolymer at Figure 5. Heat of re-crystallization (Hc) and Supercooling various temperatures with different sequences of scans. effect (T) of VDC-VC copolymer at various melting temperatures. The above discussion reveals that thermal history and degradation signiﬁcantly inﬂuence the melting and non-isothermal crystallization behavior of VDC-VC ) 1

－ copolymer, which suggests that the barrier property of △

VDC-DC may also be changed when different thermal T ( (J mol 0 c treatment is conducted. C) H

△ Isothermal Crystallization Behavior According to Avrami equation, the effect of thermal history and degradation on the isothermal crystallization behavior of VDC-VC copolymers may be studied. The relative crystallinity at time t can be expressed as: t (min) hold n Xr(t) = 1 − exp(−kt ) (1) Figure 4. Heat of re-crystallization (Hc ) and Supercooling effect (T) of VDC-VC copolymer for various holding time. where k is the rate constant, and n is the Avrami coeffi- cient which are shown in Figure 6 obtained from differ- ◦ perature is not like the regular recrystallization behav- ent Thold and recrystallization at 120 C. Equation 1 can ior, it shows a high supercooling effect at higher tem- be written as: ◦ n perature. The enhanced degree of degradation at 180 C ln(1 − Xr(t)) = −kt (2) results in a bigger difference between T and T , which m c After taking natural logarithmic calculation, eq 2 be- indicates the less ordered structure due to degradation comes: in melting state delays the recrystallization temperature − − = + even though the total crystallinity is increased. log( ln(1 Xr(t))) log k n(log(t)) (3) Figures 4 and 5 show diagrams of heat of recrys- Plot log(− ln(1 − Xr(t))) vs. log(t), the n and k values tallization (Hc) and T vs. thold and Thold during can be obtained from the slope and the intercept of the de-nucleation. These figures reveal an unusual behav- straight line. Figure 6 reveals that isothermal relative ior for recrystallization by demonstrating higher Hc recrystallization crystallinity from melting at 170◦Cis ◦ and T at higher Thold and longer thold. We under- higher than that from melting at 175 C. Because the stand longer holding time and higher holding tempera- holding time of de-nucleation at high temperatures is ture will improve the degradation and nucleation due to just 2 min, the degradation is not significant especially ◦ the formation of more nuclei of 1,3,5-trichlorobenzene at 170 and 175 C and Xr is higher at lower tempera- which would subsequently improve the crystallinity ture due to the lower degree of de-nucleation. How- ◦ with a fast rate of nucleation and increase the heat of ever, when Thold is increased to 180 C, the degradation recrystallization at the same time. This kind of be- comes into effect and Xr is increased resulting in a com- ◦ havior is different from the regular recrystallization of parable Xr values of 175 and 180 C. polymers with no degradation during melting, which Regularly, the recrystallization behavior is indepen- demonstrates lower heat of recrystallization for higher dent of holding time at melting temperature if de- melting temperature and longer holding time.10 nucleation is completed and no degradation occurs.

838 Polym. J., Vol.33, No. 11, 2001 Studies on the Crystallization of Poly(vinylidene chloride-co-vinyl chloride)

Table II. The Avrami parameters of sample under different thermal treatments a 3 r) Sample condition K(10 ) n X 170-2-100 514.3±10 2.14 170-2-110 76.4±5 2.20 170-2-120 18.7±1 2.25 170-3-100 712.2±10 2.42 170-3-110 300.6±10 2.26 170-3-120 89.8±5 2.33 170-5-100 1594.7±50 2.41 Relative Crystallinity ( Relative 170-5-110 27.2±1 2.57 170-5-120 3.1±0.5 2.34 175-3-100 6.89±1 2.34 175-3-110 1.93±0.5 2.53 Figure 6. Relative crystallinity of VDC-VC copolymer with 175-3-120 2.75±0.5 2.23 different annealing time at various melting temperatures. 180-3-90 53.4±5 2.19 180-3-100 20.0±1 2.24 180-3-110 2.63±0.5 2.37 a170-2-100 represents sample molten at 170◦C for 2 min under melt crystallization mode, and then r) ◦ X quenched to 100 C for isothermal crystallization.

thold＝1min

thold＝3min T ＝175℃ for Thold＝5min (a) t= 0 min (b) t= 2 min (c) t= 3 min (d) t= 5 min

Relative Crystallinity ( Relative

Figure 7. Relative crystallinity of VDC-VC copolymer with different annealing time at 170◦C for various melting time (thold).

◦ 2θ However, when thold is over 2 min at 170 C, cyclization degradation of VDC-VC copolymer seems to be Figure 8. Wide Angle X-Ray diffraction patterns of VDC-VC enhanced and more nuclei (1,3,5-trichlorobenzene) are copolymer after annealing at 175◦C for various time. created resulting in a higher crystallinity and rate of crystallization when recrystallization is conducted at WAXD Patterns and Degradation of VDC-VC Copoly- 120◦C as shown in Figure 7. It reveals the recrystal- mer lization behavior is very sensitive to the degradation of VDC-VC copolymer is exposed by X-Ray and the VDC-VC copolymer. diffraction patterns are shown in Figure 8, which re- These data reveal a linear relationship representing veals the less order structure of annealed samples at ◦ the presence of unique type of crystallization behavior 175 C. Some of the characteristic diffraction peaks re- during melt recrystallization. The n and k values ob- sulted from neat VDC-VC copolymer disappear after ◦ tained from various thermal treatment conditions are annealing for more than 2 min at 175 C. It is quite sure listed in Table II, which reveals an increase of aver- that the degradation is already initiated at this tempera- age n values with annealing time at lower Thold (i.e., ture, which can put an inﬂuence on the recrystallization 170◦C). It indicates the change of crystallization be- behavior at lower temperatures. havior of VDC-VC copolymer at 170◦C. The change The nucleation effect will be enhanced if more nu- is attributed by 2-dimensional re-crystallization at the clei are present during recrystallization. An unusual be- amorphous region. The average n values are all about havior of recrystallization due to degradation is found 2.33 for each sample annealed at 170, 175, and 180◦C. for VDC-VC copolymer. It demonstrates a lower crystallinity and rate of recrystallization phenomenon when annealing temperature is lower due to the nucleation effect from degradation. Usually, higher annealing temperature will have a higher crystallinity and rate of

Polym. J., Vol.33, No. 11, 2001 839 C.-L. CHEN, T.-H. HSIEH, and K.-S. HO ) r X

Tc＝ 90℃

Tc＝ 100℃

Tc＝ 110℃

Tc＝ 120℃ Relative Crystallinity ( Relative

Figure 10. Morphology of VDC-VC copolymer annealing at 180◦C for 3 min. Figure 9. Relative crystallinity of VDC-VC copolymer with different annealing time at various annealing temperatures for 5 min melting. crystallization due to diffusion controlled mechanism at high temperature. Figure 9 shows the isothermal recrystallization behavior at various annealing temperatures after melting at 170◦C for 5 min. The degraded VDC-VC copolymer owns lots of nuclei made of 1,3,5- trichlorobenzene and has an enhanced nucleation effect at lower temperature (90◦C) demonstrating a higher Xr and rate of recrystallization (slopes) regardless of higher diffusion ability at higher temperatures. Figure 11. Spherulite size of VDC-VC copolymer at different Morphology of Isothermally Annealed Crystalline annealing time at various annealing temperatures after melting at The tiny crystals, resulting from fast crystallization 175◦C for 5 min. :90◦C, : 100◦C, : 110◦C, ♦ : 120◦C. at lower Thold, cannot be observed by polarized optical microscopy. The origination of the smaller parti- At higher annealing temperature (recrystallization tem- cle size may be attributed to the presence of additive perature), the size and the growth rate (slope) is lower impurity within the copolymer matrix. Kirkpatrick8 than that at lower temperature due to the nucleation ef- pointed out the presence of impurities can increase the fect which is favorable for low temperature. Due to the nucleation rate and shorten the induction time, which copolymer structure of PVDC and PVC, the cyclization then accelerates the rate of crystallization. Similar con- of the conjugated main chains results in not only 1,3,5- clusions are drawn for VDC-VC copolymer at higher trichlorobenzene but m-dichlorobenzene (Scheme 1). ◦ Thold (i.e., 180 C) and lower crystallization tempera- Both of them are capable of behaving as a nuclei during ture (<110◦C). When temperature is higher than 110◦C, recrystallization at the following annealing at tempera- the crystallized particles can be observed isothermally ture below melting point. At high annealing tempera- ◦ due to the larger lamella thickness and slower rate of ture like 150 C as shown in Figure 11 may also cause crystallization. Figure 10 shows the picture of mor- the re-melting of nuclei made m-dichlorobenzene and phology of VDC-VC copolymer, which has been an- decrease the growth of the crystals. nealed after staying at 180◦C for 3 min. A lot of shining dots can be seen in the initial stage of crystalliza- CONCLUSION tion, which is followed by the growth of the shining dots into big spherulites. Every shining dot demon- The degradation phenomenon of VDC-VC copoly- strates spherulite structure with typical Maltese cross mer is proved by the less order X-Ray diffraction pat- patterns. The diagram of spherulite size vs. growing terns of annealed samples at 175◦C. The degraded time is shown in Figure 11, which demonstrates a lin- VDC-VC copolymer releases 1,3,5-trichlorobenzene, ear relationship. Therefore, we estimate the average which behave as the nucleus of re-crystallization dur- size of the spherulites is about 2.9 mm from the plateau ing cooling and increase the crystallinity. The super- value of the curves of 140 or 150◦C. The growth of ra- cooling effect decreases when numbers of scanning in- dial length at various annealing temperatures is simi- crease because higher N-values mean longer holding lar to that of relative crystallinity shown in Figure 9. time which can shorten the difference between Tm and 840 Polym. J., Vol.33, No. 11, 2001 Studies on the Crystallization of Poly(vinylidene chloride-co-vinyl chloride)

Tc. Higher heat of recrystallization and high degree nology”, John Wiley & Sons, Ltd., New York, N.Y. 1971, vol. of supercooling are found at higher annealing tempera- 14, p 540. tures and longer annealing time. 2. P. T. Delassus, J.Vinyl Technol., 1(1), 14 (1980). 3. P. T. Delassus and D. J. Grieser, J. Vinyl Technol., 2(3), 195 Isothermal re-crystallization crystallinity (Xc) obtained from annealing at 170◦C is higher than that at (1980). ◦ 4. M. C. Patterson and D. L. Dunkelberger, J. Vinyl Technol., 175 C. The re-crystallization of VDC-DC copolymer 16(1), 46 (1994). is very sensitive to the degradation. The degraded 5. A. Bailey and D. H. Everett, J. Polym. Sci., A-2, 7, 87 (1969). VDC-VC copolymer owns lots of nuclei made of 1,3,5- 6. G. M. Burnett and R. A. Haldon, Eur. Polym. J., 4, 83 (1968). trichlorobenzene and has an enhanced nucleation effect 7. R. A. Wessling, “Polyvinylidene Chloride”, Gordon and ◦ at lower temperature (90 C) demonstrating a higher Xc Breach Science Publishers, New York, N.Y., 1977, p 78. and rate of re-crystallization regardless of higher diffu- 8. D. E. Kirkpatrick and D. Ranck, J. Plas. Film and Sheeting, sion ability at higher temperatures. 3, 290 (1987). The polarized optical pictures reveal the growth of 9. L. Yuesheng and P. Zuren, J. Appl. Polym. Sci., 61, 2397 (1996). spherulites and the sizes are about 2.9 mm. 10. A. Jonas and R. Legras, Polymer, 32 (15), 2691 (1991). 11. T. H. Hsieh and K. S. Ho, J. Polym. Sci., Part A: Polym. Acknowledgment. This research is ﬁnancially sup- Chem., 37, 2035 (1999). ported by the National Science Council, Taiwan, 12. T. H. Hsieh and K. S. Ho, J. Polym. Sci., Part A: Polym. R.O.C. under contract NSC 83-0017-C151-00E. Chem., 37, 3269 (1999). 13. T. H. Hsieh, Polym. J., 31, 948 (1999). REFERENCES

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