Crystallization kinetics and melting behavior of metallocene short‐chain branched polyethylene fractions

Abstract

Metallocene polyethylene (mPE) fractions are recognized as being more homogeneous with respect to short-chain branch (SCB) distribution as compared with unfractionated mPEs. Differential scanning calorimetry and polarized optical microscopy (POM) were used to study the influences of SCB content on the crystallization kinetics, melting behavior, and crystal morphology of four butyl-branched mPE fractions. The parent mPE of the studied fractions was also investigated for comparative purposes. mPE fractions showed a much simpler crystallization behavior as compared with their parent mPE during the cooling experiments. The Ozawa equation was successfully used to analyze the nonisothermal crystallization kinetics of the fractions. The Ozawa exponent n decreased from about 3.5 to 2 as the temperature declined for each fraction, indicating the crystal-growth geometry changed from three-dimensional to two-dimensional. For isothermal crystallization, the fraction with a lesser SCB content exhibited a higher crystallization temperature (T c ) window. The results from the Avrami equation analysis showed the exponent n values were around 3 (with minor variation), which implied that the crystal-growth geometry is pseudo-three-dimensional. Both of the activation energies for nonisothermal and isothermal crystallization were determined for each fraction with Kissinger and Arrhenius-type equations, respectively. Double melting peaks were observed for both nonisothermally or isothermally crystallized specimens. The high-melting peak was confirmed induced via the annealing effect during heating scans. The Hoffman-Weeks plot was inapplicable in obtaining the equilibrium melting temperature (T m°) for each fraction. The relationship between T c and T m for the fractions is approximately T m ϭ T c (°C) ϩ 8.3. The POM results indicated that the crystals of parent or fractions formed under cooling conditions did not exhibit the typical spherulitic morphology as a result of the high SCB content.

Effect of long chain branching on nonisothermal crystallization behavior of polyethylenes synthesized with constrained geometry catalyst

“…In the recent years, in addition to the focuses on the chain structure and processing property relationships [23, 25–27], a great deal of attentions have been paid to investigating the influence of chain structure on crystallization kinetics of polymers and their blends. Chiu et al [2, 28, 29] studied the effect of short chain branching (SCB) on crystallization kinetics and crystal morphology of metallocene polyethylenes having well‐controlled molecular weight, and found both the onset and peak cooling temperatures shifted toward lower temperatures with increasing SCB densities (SCBD) during the cooling period, which was also confirmed by Islam et al [30]. High SCBD resulted in a reduced length of crystallizable ethylene sequence.…”

“…Polyethylene (PE) is one of the largest polymer products in the global market. Various grades of PEs have become commercially available with the aid of substantial developments in catalysis technologies to meet the growing variety of applications in modern industries [1–4]. The chain topology, branched structure, and molecular weight and distribution, which determine the physical, mechanical, and rheological properties of PEs, can be controlled using different catalyst systems to tailor make the materials [5–9].…”

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“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The process is conducted by cooling the melt at a controlled cooling rate. By using differential scanning calorimetry (DSC) and/or differential thermal analysis (DTA) one readily determines the values of T p as "peak temperatures".…”

Estudo experimental do processo de rotomoldagem de PELBD: efeitos sobre a morfologia e estabilidade dimensional

“…Por outro lado, taxas de resfriamento mais brandas promovem uma menor densidade de nucleação resultando em uma microestrutura com esferulitos maiores e lamelas mais espessas [30,31] . Observam-se através das Figuras 5 e 6 a presença de algumas bolhas nas amostras rotomoldadas em diferentes condições de processamento.…”

“…Para eliminar totalmente as bolhas dos artigos rotomoldados seriam necessários longos tempos de ciclo O desenvolvimento da fase cristalina está intimamente ligado à taxa de resfriamento. Dessa maneira, quanto maior a taxa de resfriamento menor será a formação da estrutura cristalina no polímero [30,31] . Os resultados obtidos por DSC estão de acordo com as micrografias obtidas para estas amostras onde as regiões submetidas a taxas de resfriamento mais brandas apresentaram resistência ao momento fletor desenvolvido e menor será o nível de empenamento.…”