Rheological Evidence of Physical Cross-Links and Their Impact in Modified Polypropylene Yan Li, ‡,§,∥ Zhen Yao,* ,‡ Zhen-hua Chen, §,∥ Shao-long Qiu, ‡ Changchun Zeng,* ,§,∥ and Kun Cao* ,†,‡ † State Key Laboratory of Chemical Engineering and ‡ Institute of Polymerization and Polymer Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China § High Performance Materials Institute, Florida State University, Tallahassee, Florida 32310, United States ∥ Department of Industrial and Manufacturing Engineering, FAMU−FSU College of Engineering, Tallahassee, Florida 32310, United States ABSTRACT: This paper reports our investigation of the existence of physical cross-links in modified polypropylenes (PPs) containing long chain branches (LCBPPs) or amine moiety (PP-g-NH 2 ). By varying the stoichiometric ratio of maleic anhydride grafted polypropylene (PP-g-MAH) and ethylenediamine (EDA), a series of modified PPs with different degrees of branching and side-group polarities were prepared. Extensive rheological studies were conducted after baseline characterization of the chemical and molecular structures of these materials using Fourier transform infrared (FTIR) spectroscopy and size exclusion chromatography (SEC), respectively. The results strongly suggest the presence of physical cross-links in a majority of the materials studies herein, which significantly impacts their rheological behaviors. The physical cross-links can be argued to be in the form of phase-separated domains and hydrogen bonding, which has been reported in the literature. 1. INTRODUCTION Isotactic polypropylene (iPP) is one of the leading and fastest- growing polyolefins because of its attractive properties, such as high melting point, low density, excellent chemical resistance, and high tensile strength. 1 However, linear PPs possess low melt strength, which limits their use in processes involving substantial elongational flow such as thermoforming, film blowing, extrusion coating, blow molding, and foaming. 1,2 Introducing long chain branching (LCB) into the molecular structure of PP, through either in-reactor polymerization or postreactor treatment, has proven effective to overcome this shortcoming. 2−11 Among the approaches for LCB introduction, reactive coupling of PP chains by small linker molecules 11,12 is advantageous in several aspects, e.g., easy implementation and flexibility in controlling the branching structure. 13−16 Taking advantage of the high reactivity of the imidization reaction, a number of research groups prepared long chain branched PPs (LCBPPs) using amine and maleic anhydride grafted PP (PP-g- MAH). 6−11 By varying the NH 2 /MAH ratio R, the structure of the PPs, e.g., molecular weight, branching degree, and density of the function groups, can be tailored. 6,9,17 Aside from the topological change, the imidization reaction may also alter the local polarity within the molecules as a result of the incorporation of high polarity linkages. The disparity in polarity may lead to phase separation, and the phase-separated domains may serve as physical cross-links leading to potentially significant changes of mechanical and rheological properties of these materials. To the best of our knowledge, these issues have not been discussed in the literature, although physical cross- links have been observed in several polymers containing highly polar groups, e.g. hydroxyl, 18,19 carboxyl, 20−22 anhydride, 23,24 and ionic groups, 25 and the influences of such structures were well documented. More recently, formation of inhomogeneity or microphase separation by grafting a polar monomer (pentaerythritol triacrylate, PETA) onto another polymer, e.g., polylactic acid (PLA), had also been reported. Although not directly justi fied from rheology, its influence on crystallization suggests the existence of inhomogeneity. 26 In this work, we set out to investigate the possible presence of the phase-separated structures in the modified PPs, their formation, and their impact on rheological properties. The employed samples were a series of LCBPPs prepared by supercritical carbon dioxide (scCO 2 ) assisted reactive extrusion of PP-g-MAH and ethylenediamine with varying ratios for controlling the extent of reaction and degradation, 15 and an amine grafted polypropylene (PP-g-NH 2 ) prepared via a solution process. 27 The study was divided into two sections: (i) detailed structural characterizations were conducted to examine the major differences between the modified samples (molecular weight, molecular weight distribution, gel content, etc.), which form the basis to deconvolute various factors when interpreting the rheological response of these materials; (ii) both linear and nonlinear shear rheometry and extension rheometry were then conducted, and the results strongly suggest that the rheological behavior of the modified PPs can be affected by both long chain branches (LCBs) and physical cross-links present. 2. EXPERIMENTAL SECTION 2.1. Materials. PP-g-MAH (MAH content, 0.3 wt %) was from Ningbo Nengzhiguang New Material Co., Ltd., China. Ethylenediamine (EDA) was purchased from Hangzhou Changqing Chemical Reagent Co., Ltd., China. 1,2,4- Received: March 12, 2013 Revised: April 18, 2013 Accepted: May 15, 2013 Published: May 15, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 7758 dx.doi.org/10.1021/ie400809z | Ind. Eng. Chem. Res. 2013, 52, 7758−7767