The high temperature secondary crystallisation of

Polymer Testing 23 (2004) 621–627 www.elsevier.com/locate/polytest

Material Behavior

The high temperature secondary crystallisation of aged isotactic polypropylene D. Dudic´, V. Djokovic´ , D. Kostoski ‘‘Vincˇa’’ Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia and Montenegro Received 22 December 2003; accepted 30 January 2004

Abstract The secondary crystallization of aged isotactic polypropylene (iPP) of different morphologies, which occurs during v annealing in the vicinity of the melting temperature (140 C), was studied using differential scanning calorimetry (DSC) and density measurements. The results of calorimetric and density measurements were correlated. Kinetics of the high temperature secondary crystallization (HTSC) was studied by using Avrami relations. Avrami exponents, v obtained according to the increase in crystallinity after annealing at 140 C, were considered as the measure of the ability of iPP to crystallize secondarily. Depending on the initial morphology of the samples, accelerated ageing influences in different ways the ability of iPP to subsequently crystallize upon annealing. The aged samples with larger amorphous content show a reduced secondary crystallization at high temperature compared with the unaged samples. On the other hand, aging of medium crystalline samples leads to more pronounced HTSC. # 2004 Elsevier Ltd. All rights reserved. Keywords: Isotactic polypropylene; Secondary crystallization; DSC; Ageing; Avrami exponents

1. Introduction Application of polymers depends significantly on the ability of their structure to stay unchanged as long as possible. Recrystallization processes as well as changes in the amorphous fraction of semicrystalline polymers (such as iPP), induced by ageing, can alter the properties of the initial material [1–8]. In the case of physical ageing at room temperature, it was found that the main effects were connected with the amorphous phase [1,5,6]. Also, despite the lowering of the amorphous free volume during 250 days of physical ageing at room temperature, Yue and Msuya [6] have not noticed the secondary crystallization or lamellae thickening in iPP. In most practical cases, accelerated ageing, i.e. application of high pressure and elevated temperature,

is used instead of physical ageing in the investigations of long-term structural changes. However, in our previous paper [8] we established that this treatment could induce secondary crystallization of iPP. Such a behavior is the consequence of oxidative chain scissions and possible slide of non-scission tie molecules despite the presence of antioxidant additives in the material. In the present paper, we have made one step further through the investigation of the high temperature secondary crystallization (HTSC) of the already aged iPP samples with different initial morphology. Using the results from DSC and density measurements, the kinetics of the HTSC was studied via the Avrami relations. The other part of the study concerns the well-known discrepancy between the crystallinities determined from DSC and density results. An effort is made to treat this problem through mathematical correlation. 2. Experimental

Corresponding author. Tel.: +381-11-245-3986; fax: +38111-344-0100. E-mail address: [email protected] (V. Djokovic´).

The commercial stabilized iPP NESTE type VC W ¼ 136 kg mol1 and d ¼ 0:908 g cm3) 1064 K (M

0142-9418/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.01.015

D. Dudic´ et al. / Polymer Testing 23 (2004) 621–627

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containing antioxidant additives was used. Isotropic sheets 0.5 mm thick were prepared by compression v molding in a Carver laboratory press at 200 C and 1.75 MPa. The molded sheets were cooled at three different cooling rates: quenched in ice-water (A); cooled in air (B); slowly cooled in the press (C). The process v of accelerated ageing was performed at 60 C in an Emmerson single vessel oxygen apparatus at 0.5 MPa. The secondary crystallization of unaged and aged samv ples was carried out by annealing at 140 C in an inert atmosphere. A Perkin-Elmer DSC 2 differential scanning calorimeter fitted with a data acquisition system and original software was used. Samples of 5–6 mg were analyzed in nitrogen flow by heating (20 K min1) v from 35 to 190 C. Density measurements were performed by the denv sity gradient column technique at 23 C. The mass crystallinity was calculated from the density measurements as d ¼ XM

dC d dA d dC dA

ð1Þ

where d is the measured value of density, whereas dC and dA are the densities of the crystal and amorphous phase, respectively. From DSC measurements, the mass crystallinity was calculated as c ¼ XM

D Hf D Hm

ð2Þ

cesses as accurately as possible, we correlated the crystallinities obtained from the density and DSC measurements. The idea was to find out the values of the parameters dA, dC and DHm, which would lead to the minimal discrepancy between the mass crystald c linities XM and XM of iPP with different morphologies (samples A, B and C). Those parameters were after that used to study the kinetics of the secondary crystallization of iPP. Mathematical details of the correlation procedure are presented in Appendix A of this paper. Briefly, mathematical analysis suggested that for the correlation it was necessary to keep one of the parameters fixed. However, since all the three quantities dA, dC, and DHm are thermodynamically correlated, choosing any of them to be constant is debatable. Nevertheless, we decided to take the density of the crystal phase as a constant during the estimation. In our samples, the monoclinic-a phase is the dominant crystalline structure and the value dC ¼ 0:936 g cm3 is related to the type of the crystal cell. Crystallinity correlation with a fixed crystal phase density led us to the following values: dA ¼ 0:8693 g cm3 and D Hm ¼ 161 J g1 . The d c mean square error between XM and XM , calculated by using the above parameter values in Eqs. (1) and (2), is about 1.6% of the absolute value of crystallinity. 3.2. Secondary crystallization induced by ageing

where DHf is the measured melting enthalpy and DHm is the enthalpy of fusion of a 100% crystalline polypropylene at equilibrium melting point.

The changes of the volume crystallinity of all the three types of the samples (A, B and C), induced by accelerated ageing, are shown in Fig. 2. The volume crystallinities were estimated from the following equation

3. Results and discussion

XVd ¼

3.1. Crystallinity correlation Fig. 1 shows mass crystallinities of the unaged sample B, obtained by DSC and density measurements, as v a function of annealing time at 140 C. The crystallinites are calculated by using D Hm ¼ 209 J g1 , dA ¼ 0:856 g cm3 and dC ¼ 0:936 g cm3 [9] in Eqs. (1) and (2). As can be seen in Fig. 1, both methods suggest an increase in crystallinity with increasing annealing time. However, the crystallinity values estimated according to the results of density measurements are at least 20% higher than the values estimated according to DSC. Similar results are also obtained for the samples A and C. There are several reasons for such large discrepancies in crystallinity [10]. The most probable may be the fact that, contrary to density, the measured value of fusion enthalpy was obtained from a thermodynamically non-equilibrium process. Because of that, in order to describe the secondary crystallization pro-

d dA ; dC dA

ð3Þ

using the correlated density of the total amorphous iPP, dA ¼ 0:8693 g cm3 . It should be noted that both methods, the DSC and density measurements, separately verify the results in Fig. 2. It can be seen in Fig. 2 that, depending on the initial morphology of the samples, crystallinity changes induced by ageing will be different. After a 24 h of ageing, the quenched sample (A) has higher crystallinity than the unaged one. This is a v result of recrystallization of the smectic phase at 60 C [11]. However, prolonged ageing of this sample up to 72 h decreases its crystallinity. The crystallinity of the sample B decreases steadily with ageing time. The decreasing of crystallinity means that the ageing of sample B, as well as the prolonged ageing of sample A, induces degradation of the lamellae (probably at the crystallite borders). On the other hand, the crystallinity of sample C increases with increasing ageing time. This is a consequence of a large number of taut tie molecules in the sample with the highest crystallinity. These mole-


623

Fig. 1. Mass crystallinity of samples B vs. annealing time (140 C), obtained by DSC ( D Hm ¼ 209 J g1 [9]) and density measurements (dA ¼ 0:856 g cm3 and dC ¼ 0:936 g cm3 [9]). v

cules are more liable to oxidative chain scissions, which consequently cause the reorganization of the crystal phase and the relief of the strained parts of the lamellae [8]. Increasing of the lamellae perfection can be well seen by the increase of the melting temperatures of sample C after 24 h of ageing (Fig. 3a). Fig. 3a also shows that ageing did not affect the melting peak temperatures of the A and B samples. The former results indicate that, depending on the initial morphology, the recrystallization processes of iPP during ageing will be different. It will be seen in the next section that the kinetics of the HTSC of iPP also depends on the ageing effects in the samples prior to annealing.

v

of annealing at 140 C. As a result of annealing, i.e. HTSC, an increase of the melting temperature as well as narrowing of the lamellae thickness distribution of all the samples can be noticed. In Fig. 3b it can be also seen that, after HTSC, the aged A sample has a lower melting temperature compared to the unaged sample. On the contrary, annealing produces increase in melting temperatures of the aged B and C samples with respect to the unaged samples (Fig. 3b). In order to elucidate the influence of ageing history and morphology on the HTSC of iPP, the Avrami equation is used [12]. Avrami’s equation is given as n

XV ðtÞ ¼ 1 eKt ; 3.3. Kinetics of the high temperature secondary crystallization Fig. 3 depicts the melting curves of the aged and unaged A, B and C samples before and after 1400 min

Fig. 2.

ð4Þ

where XV is volume crystallinity, K is the characteristic constant, t is the time of crystal growth and n is an Avrami exponent. The Avrami exponent n depends on the type of nucleation and geometry of crystal growth.

Crystallinity of the samples vs. ageing time. Size of the symbols corresponds to experimental error.

624


v

v

Fig. 3. Melting curves of the samples A, B and C: (a) before annealing at 140 C, (b) after annealing for 1400 min at 140 C. Solid line—unaged, dashed line—aged for 24 h.

It can also be treated as a measure of the ability of the material to crystallize secondarily. The Avrami exponents were determined, according to the increase of volv ume crystallinity (Eq. (3)) during annealing at 140 C, from the slope of the function logfln½1 XV ðtÞg vs. log t. The Avrami plots for the unaged A, B and C samples are shown in Fig. 4. The obtained Avrami exponents (n) are given in Fig. 5 as a function of ageing time. It can clearly be seen that the initial morphology and recrystallization processes during accelerated ageing induce different behavior of the material upon subsequent annealing. As expected, the unaged sample A, obtained by quenching from the melt, has the highest n because of the lowest crystallinity. However, after ageing, n of this sample decreases, which implies a lower ability of the material to crystallize again. This could be a consequence of the reduced chain mobility in the amorphous phase of the polymer due to ageing, as it was mentioned in the introduction. As a result of the reduced

chain mobility, the thickening of the lamellae will be less pronounced (Fig. 3b shows that the aged A sample has a lower melting temperature). Also, because of the presence of the smectic phase in the quenched samples, some of the recrystallization processes occurred during v the ageing at 60 C. The increase of crystallinity of the sample A after the 24 h of ageing supports the former conclusion (Fig. 2). On the other hand, increasing of the Avrami exponents values with ageing time was observed for B samples. Despite the reduced chain mobility in the amorphous phase, this result implies that in the B samples HTSC is favored after accelerated ageing. Oxidative chain scissions are probably responsible for such behavior of the samples cooled in air. Chain scissions create better conditions for the secondary crystallization because the broken chains are able to crystallize on the already existing lamellae, producing their thickening. This is confirmed by the higher melting temperature of the aged B sample after annealing, compared with the unaged one (Fig. 3b).

Fig. 4. Avrami plots for the unaged A, B and C samples.


Fig. 5.

625

Avrami exponents of the A, B and C samples vs. ageing time.

The samples obtained by slow cooling in the press, type C, have the smallest Avrami exponents, which can be attributed to their highest crystallinity (Fig. 2). In contrast to the samples cooled from the melt in air, the Avrami exponents slightly decrease with ageing time. However, the changes are within the experimental uncertainty, which suggests that the influence of ageing history on the Avrami exponents is practically negligible (Fig. 5). The fact that most of the crystallization processes were finished by slow cooling in the press could be a reason for negligible effects of ageing on subsequent crystallization upon annealing of the sample C. As discussed in the previous section, the ageing itself induces a slight increase in the crystallinity and melting temperature of this sample due to enhanced number of oxidative chain scissions. After that, the main effect of annealing is the increase of the lamellae perfection, which can be seen in Fig. 3b via an increase v of the melting temperature of about 3 C. 4. Conclusion The present results show different kinetics of high temperature secondary crystallization of aged and unaged iPP. The kinetics of the secondary crystallization during the annealing is followed via changes in the Avrami coefficients. It was found that the highest crystalline samples (C) have the lowest Avrami coefficient, which is independent of the ageing history because most of the recrystallization processes are finished during the slow cooling in the press. After ageing, the high temperature secondary crystallization is favored in the samples with medium crystallinity (B). Quenched samples show a behavior different from that of the samples B and C. The changes in the morphology of the sample A, due to ageing, reduce its ability to crystallize secondarily.

Appendix A. For correlation of the crystallinities obtained by DSC and density measurements, N ¼ 20, different iPP samples of all morphologies have been used in the present paper (as prepared, aged, unaged and annealed A, B, C samples). Their densities and melting enthalpies were in the intervals 0.9–0.92 g cm3 and 81–111 J g1, respectively. Mathematical procedure for the crystallinity correlation is based on the determination of a minimum of the function D(dA, dC, DHm) given by

DðdA ;dC ; DHm Þ ¼

N X

d c XMi XMi

i¼1

¼

N X 1 ðdA =di Þ i¼1

1 ðdA =dC Þ

D Hfi D Hm

2

2 ðA:1Þ

d c where XM and XM are mass crystallinities defined by Eqs. (1) and (2). Stationary points are found by solving the following system

@D @D @D ¼ 0; ¼ 0; ¼ 0: @dA @dC @ D Hm

ðA:2Þ

Solutions of the Eq. (A.2) system are given by dA ¼

B ¼ 0:8719 g cm3 ; A

ðA:3Þ

dC ¼

dA B þ C ¼ 1; dA A þ B

ðA:4Þ

N X 1 ðdA =dC Þ D Hfi2 PN i¼1 Hfi dA i¼1 ðDHfi =di Þ i¼1

D Hm ¼ PN

¼ 2420 J g1 ;

ðA:5Þ


626

d Fig. 6. Minimal values of the function D(dA, dC, hHm) (given in the form of the mean square crystallinity deviation between XM c and XM ) obtained using seria of dC ¼ constant values. Above the specific points are the corresponding values for dA and hHm.

where A, B and C are quantities obtained from experimental results according to the following formulas: !2 N N N X X 1 X D Hfi 2 D Hfi ; A¼ di d2 i¼1 i i¼1 i¼1 N N N X D Hfi X 1 X D Hfi2 ; d d i 1 i¼1 i¼1 i¼1 i¼1 !2 N N X X C¼N D Hfi2 D Hfi :

B¼

N X

D Hfi

i¼1

ðA:6Þ

i¼1

Taking into account the breaking points dA ¼ dC in Eq. (A.2) system, which are not interesting to us from the physical point of view, and the obtained solutions, we can claim that the function D(dA, dC, hHm) does not have local minima. Further, in order to get a minimum of the function D(dA, dC, hHm), it was concluded that it was necessary to keep one of the arguments constant during the estimations, in this case the crystal density. Fig. 6 shows the minimal values of the function D(dA, dC, hHm), represented in the form of mean d and square deviation between the crystallinities XMi c XMi (i ¼ 1, 2,. . .N), which is obtained by using series of different dC ¼ const: values. The reason why the crystal density is chosen as a constant is its relation to the type of crystal cell. Therefore, we decided to choose the value dC ¼ 0:936 g cm3 [9], which is related to the dominant crystal structure in our samples, i.e. the monoclinic a phase. Although the total measured melting enthalpies are mainly determined by the sample crystallinity, they also depend on the thermodynamic state of the amorphous

phase and surface effects of the crystals. On the other hand, the amorphous density, dA, depends on the ordering of the amorphous phase and the presence of additives. Thus, it was assumed that taking parameters dA and hHm as constant during correlation was not suitable. By solving the system @D @D ¼ 0; dC ¼ 0:936 g=cm3 ; ¼ 0; @dA @ D Hm

ðA:7Þ

which is obtained by including the value dC ¼ 0:936 g cm3 into the system given by Eq. (A.2), correlated values for dA, dA ¼

dC B þ C ¼ 0:8693 g=cm3 dC A þ B

ðA:8Þ

and hHm N X 1 ðdA =dC Þ PN i¼1 Hfi dA i¼1 ð D Hfi =di Þ i¼1

D Hm ¼ PN

D Hfi2 ¼ 161 J g1

ðA:9Þ

are estimated. Using the above correlated values in Eqs. (1) and (2), the mean square error between the d c mass crystallinities XM and XM is about 1.6% of crystallinity. References [1] V. Vittoria, Investigation of ageing of isotactic polypropylene via transport properties, Polymer 29 (1988) 1118. [2] J.B. Knight, P.D. Calvert, N.C. Billinghem, Localization of oxidation in polypropylene, Polymer 26 (1985) 1713.

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