Quiescent and stress induced crystallization studies of nucleated and nonnucleated polyolefins

Author

Hari P. Nadella

Date of Award

6-1983

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Polymer Engineering

Major Professor

Joseph E. Spruiell

Committee Members

James L. White, Edward S. Clark, D. C. Bogue

Abstract

Isothermal crystallization studies of nucleated and nonnucleated polypropylene and high density polyethylene were conducted under quiescent and stress induced conditions (SIC). For SIC case, the samples were subjected to both elongational and shear flow deformations on a Rheometrics Mechanical Spectrometer over a range of crystallization temperatures. Changes in the sample viscosities were recorded as function of time at constant temperature and deformation rate. Induction times, defined as the time from start of deformation to the point where they increase from a steady state value, were obtained. These induction times were interpreted as proportional to the nucleation rates. In the case of quiescent crystallization studies, the differential scanning calorimeter was used and the sample exotherms were recorded under isothermal conditions.

Quiescent crystallization results were analyzed using the Avrami approach. The results showed an increase in the crystallization rates with decreasing temperature in both control and nucleated polypropylenes and polyethylene. Avrami index was found to be approximately 3 for all the samples. Under quiescent conditions, addition of 0.5 wt % of sodium benzoate nucleating agent to poly propylene increased the maximum crystallization temperature by about 20°C over the control. The talc nucleated sample crystallized only at slightly higher temperature. The effect of sodium benzoate concentration was nonlinear and seems to saturate as the concentration is increased.

SIC results in polypropylenes and polyethylenes showed increases in nucleation rates with increase in the stress (shear or tensile) and decrease in crystallization temperature. In polypropylene at low shear rates (γ ≤ 0.1 sec^-1) the nucleation rates approached the quiescent values at higher crystallization temperatures. Nucleated polypropylenes under shear flow conditions showed additional (compared to sheared control sample) enhancement in their nucleation rates. Depending on the temperature at which the sample is being stressed, addition of nucleating agent (at γ = 0.1 sec^-1) increased the maximum crystallization temperature by 7 to 19°C (overall increase in Tcr,_Max 44°C over control quiescent case). Expressions for SIC relating nucleation rates to TrP were successfully developed using classical nucleation theories following Jarecki and Ziabicki's analysis. In addition an equation relating nucleation rate to molecular orientation in the melt was proposed from intuitive arguments. SIC results were analyzed using the above two approaches. In both polypropylene and polyethylene the differences in the nucleation rates between the samples crystallized under shear and elongational flow are significant when the nucleation results for a given crystallization temperature were plotted as a function of stress (shear or tensile). Replots of the same results as function of TrP or σ_x - σ_y showed a unique correlation independent of mode of deformation but dependent on the sample molecular weight distribution and temperature. This suggests that the high molecular weight species in the broader distribution samples are important in determining the crystallization behavior of the sample.

Predictions of nucleation rates from theoretical considerations showed similar trends and temperature sensitivity as the experimental results. However, they were lower than the experimental values for given ΔG_def due to under predictions in ΔG_def (ΔG_def = ½ TrP) in Marrucci's analysis. The results also showed overprediction of nucleation rates when the rubber elasticity theory (affine deformation) was used to estimate ΔS_def.

Our experimental results were successfully curve fitted by a simplified Jarecki and Ziabicki relation. The constant a₁₁(ΔG_def = σ₁₁ TrP) ranged from 50 to 80 and was found to be dependent on the sample molecular weight distributions.

Recommended Citation

Nadella, Hari P., "Quiescent and stress induced crystallization studies of nucleated and nonnucleated polyolefins. " PhD diss., University of Tennessee, 1983.
https://trace.tennessee.edu/utk_graddiss/13111