Abstract The use of controlled extension flows associated with processing with predominant shear flows can bring gains in the dispersive capacity of nanofillers during the processing of nanocomposites. In this work, we use a controlled flow device coupled to the matrix of a single-screw extruder to process PP matrix composites containing 0.5% and 2.5% (v/v) of MWCNT. The nanocomposites were evaluated by optical microscopy and oscillatory rheological analysis. The clusters size analysis showed that in PP/0.5MWCNT-el, 70.5% of all its clusters were below 1µm2 while its analog processed without the extensional flow showed 59.8% of its clusters below this value. The rheological analysis allowed to verify that the compositions processed with the presence of the extensional flow have their crossover frequencies shifted to lower values, that is, longer relaxation times corroborating that improved degrees of dispersion were achieved.

Abstract The structure and properties of semi-crystalline polymers can be drastically tailored by extensional flows. In this work, polypropylene (PP), Polyvinylidene fluoride (PVDF) and Low Density Polyethylene (LDPE) were melt extruded through a sequence of rings designed to apply controlled extensional flows in the polymer melts. The effects of extensional flow on the structure and properties of the extruded filaments were then evaluated by mechanical tensile tests, dynamic-mechanical analysis (DMA) and Differential Scanning Calorimetry (DSC). The DMA and tensile tests revealed a significant increase in terms of static and dynamic moduli for the polymers extruded through the extensional flow device. PP, PVDF and LDPE had their dynamic moduli enhanced 19%, 40% and 77%, respectively. These results were ascribed to the enhancement in crystallinity and orientation degree of the polymer chains induced by the extensional flow. The crystallinity was increased around 9% for PP, PVDF and LDPE extruded under extensional conditions.

The purpose of the present paper was to test the validity of the nonlinear regression method for calculating the non-isothermal crystallization rate constant of the Nakamura's model of a rotational molding grade LLDPE directly from non-isothermal crystallization experiments carried out in a single cell DSC. Cooling rates of 50, 40, 30, 20, 10 and 5 oC/min were used with samples of 3.0 mg under nitrogen atmosphere. Here, good agreement was observed between the experimental relative crystallinity curves and the simulated ones using the calculated parameters by nonlinear regression. It shows that this method can be used to determine the Nakamura's non isothermal rate constant for using in simulation of the cooling phase of rotational molding. In this paper it was used 10-3, 10-4 and 10-14 as the initial crystallinity in the Nakamura's model. However the best average results for all cooling rates was obtained when 10-4 was used. Average spherulitic dimensions of LLDPE studied in this paper did not change significantly with different cooling conditions.

No presente estudo o empenamento de peças de polietileno linear de baixa densidade (PELBD) moldadas por rotomoldagem foi investigado. O efeito de diferentes fatores como espessura da peça, taxa de resfriamento e diâmetro de tubo de ventilação foi avaliado. Além dos experimentos de rotomoldagem, uma técnica alternativa denominada "Hot Press" foi também aplicada para investigar o empenamento de dois diferentes PELBD sob condições típicas da rotomoldagem quanto ao resfriamento assimétrico. Cristalinidade e morfologia esferulítica ao longo da espessura das peças rotomoldadas foram avaliadas por Calorimetria Exploratória Diferencial (DSC) e Microscopia Ótica de Luz Polarizada. Verificou-se que o grau de empenamento aumenta com a taxa de resfriamento. O aumento do diâmetro do tubo de ventilação é mais efetivo na redução do empenamento principalmente em menores taxas de resfriamento. Nos experimentos de "Hot Press" o PELBD com menor índice de fluidez e maior módulo de flexão apresentou menor empenamento. Nos experimentos de "Hot Press" a espessura das peças afetou o empenamento somente para maiores taxas de resfriamento. Em geral, a cristalinidade e o tamanho de esferulitos se mostraram menores nas posições ao longo da espessura das peças rotomodadas onde as taxas de resfriamento eram mais rápidas.

In the present study the warpage of rotationally molded parts of a linear low density polyethylene (LLDPE) was investigated. The effect from different processing factors such as part thickness, cooling condition and diameter of the venting tube was evaluated. In addition to the rotational molding experiments, an alternative experimental technique, referred to as "Hot Press", was also applied to investigate the warpage of two different grades of LLDPE under typical rotational molding conditions, which means slow cooling from only one side. The crystallinity and spherulitic morphology along the thickness of the rotationally molded parts were studied by Differential Scanning Calorimetry and Polarized Light Optical Microscopy. It is shown that the warpage increases with increasing cooling rate. The increase in diameter of the venting tube is more effective in reducing the warpage of rotationally molded parts, especially for lower cooling rates. In hot press experiments the grade of LLDPE with lower melt flow index and higher flexural strength presented lower warpage. The part thickness affects the warpage in hot press experiments only for faster cooling rates. In general, crystallinity and spherulitic diameter are lower in positions along the cross section of the rotationally molded part where the cooling rate is faster.

In the present work, non-isothermal crystallization data in the form of heat flow vs. temperature curves were generated using the Nakamura Model and its typical parameters reported in the literature for polypropylene. The values obtained for these curves were added to experimental baselines of a DSC to introduce typical noises in the calorimetric traces generated. The Master Curve Approach was applied by one of the authors to retrieve the non-isothermal crystallization rate constant for these simulated curves without information about the conditions used to generate these pseudo-experimental curves. Thus, the applicability of the Master Curve Approach was tested for data perfectly described by the Nakamura Model. With this procedure, the authors were able to check the sensitivity of the method to uncertainties in the determination of the induction time. The results showed good agreement between the pseudo-experimental curves and the curves simulated using the retrieved non-isothermal crystallization rate constant. However, this good agreement was only possible due to a compensation effect, because some of the parameters showed significant differences between the pseudo-experimental and retrieved values. Among these parameters are the pre-exponential factor of the non-isothermal crystallization rate constant, the temperature of the onset of the crystallization process, and the initial degree of crystallinity of the differential form of the Nakamura Model. These problems were not caused by the Master Curve Approach, but by inherent difficulties in the DSC analysis.

Due to the relevance of the critical cooling rate, Rc, for glasses, Barandiarán and Colmenero (BC) developed a method for calculating Rc as a function of the crystallization temperature on cooling obtained from thermal analysis. The critical cooling rate is obtained by the extrapolation method to conditions of infinity undercooling. However, for polymers, there is a strong reason for modifying the original BC method. In this case, the extrapolation must be extended only to the undercooling associated to the glass transition temperature, Tg, because no crystallization can occur below this temperature. Following this modified method (MBC) proposed by the present authors, the critical cooling rate for PP, PEEK, P10MS and PET were determined. The results showed that the new values are much lower than those obtained by the original BC method.