A detailed survey for determination of the grafted semifluorinated acrylic compound effect on thermal, microstructural, free volume, mechanical and morphological features of HDPE

Highlights

• Side chain graft copolymerization of the fluoroalky acrylate onto HDPE was performed with high yield.

• Impressive mechanical reinforcement was obtained by using the flouroalkyl acrylate including perfluorinated pendant units.

• The grafting onto HDPE caused the linearly increament in the free volumes in consistence with the percent poly(ABCF13).

• Significant changes in a and b unit cell parameters as well as particle size of HDPE crystals with grafting were recorded.

• The graft copolymerization brought about the significant increments in the melting temperature of HDPE crystalline domains.

Abstract

This study mainly aims to investigate the role of grafted semifluorinated acrylic compound in the crucial characteristic properties of high density polyethylene (HDPE). Graft copolymerization of the semifluorinated acrylic compound, 3,3,4,4,5,5,6,6,7,7,8,8,8-dodecafluoro-5-methyloctyl-4-(acryloyloxy) benzoate (ABCF13) onto HDPE within the different content levels of ABCF13 (5, 10, 15, 20, 30 and 40 %) was performed by bulk-melt polymerization method. The changes in the thermal, microstructural and free volume characteristics of graft coproducts with extent of grafting were investigated in detail by means of DSC, powder X-ray diffraction and PALS techniques, respectively. The experimental findings showed that the melting temperature of crystalline domains and crystallinity ofHDPE reached the maxima at the 15 % and 40 % ABCF13 contents, respectively. The grafting of ABCF13 onto HDPE also gave rise to significant expansion and contraction in lateral dimensions of orthorhombic unit cell parameters in the crystalline domains of HDPE. Moreover, PALS analyses depicted that the size of the free volumes increased almost linearly with the grafting percentage. As for mechanical behaviours of the graft products, remarkable improvements were achieved in especially tensile strength (from 18.75 MPa to 43.75 MPa) and impact strength (from 19.30 Mpa to 38.72 MPa). The morphological properties of the fractured surfaces obtained from tensile and impact tests of products were also examined by SEM analysis. The samples possessed completely homogeneous structure without any phase separation. A gradual transition from ductile behaviour to brittle nature was observed as the grafting level increased in the products.

Introduction

The most common way to upgrade the crucial some characteristics of thermoplastics (TPs) is the preparation of their blends formed by combining with rigid-rod like polymers such as liquid crystalline polymers. In many studies, the rigid-rod like polymers, exhibiting relatively considerable larger mechanical performance thanks to their stiff molecular backbones with the ability of high orientation, have been blended with TPs so as to obtain reinforced composite like materials [[1], [2], [3]]. In order to achieve effective mechanical improvements in these blends, it is required that the dispersed polymers having rigid backbone in the molecular structure act as fibrils in the TPs matrix. The fibrillar structure of the rigid-rod like polymer component strengthen the TPs matrix like in the fiber-reinforced polymer composites due to the fact that these fibrils forming under the proper processing conditions possess the relatively larger load carrying capacity [4,5].

It is known that the fluorine-containing polymers have continuously growing role in the materials science due to their superior properties. The substituting fluorine for hydrogen in a number of fully or partially fluorinated polymer molecules cause three main useful corridors: (I) increase in thermal stability and reduction in flammability (II) lower surface energy, supporting anti-adhesiveness, lower friction coefficient, self-lubricating and (III) impressive optical and electrical properties [6]. Despite of these advantages, the practical usage of fluorine containing polymers in the TPs blends are highly restricted due to the serious compatibility trouble between components, which resulting in phase separation in the polymer matrix. It is well known the fact that fully or partially miscible systems in the blends gain better properties to the polymer matrix compared with totally immiscible systems [7,8]. Thus, in order to ensure homogeneity without any phase separation, the certain amount of the compatibilizing agents such as catalyst [9], modified graft [10,11], block [12], liquid crystal [13] or any polymer [14] are used in the matrix of thermoplastic polymer blends. In addition to that, the another effective way to obtain fluorine-containing polymer/TPs blend having desired and improved properties is the designing of copolymers cooperated with more fluorinated groups or grafting of the modified partially (or fully) fluorinated monomers onto thermoplastics [[15], [16], [17]]. With this aspect, since fluorooalkyl acrylates (FAs) possesses the rigid perfluorinated pendant unit in the helix structure, self-organization ability in the matrix, remarkable low surface energy caused by the fluorine atoms, chain sliding effect and tendency to show smectic phase due to the fluorinated groups, the designed FAs could serve as the component of the thermoplastic polymer blends to achieve the improvements in the polymers [[18], [19], [20], [21]]. In a way that supports this approach, in terms of mechanical characters, the presence of the rigid-rod like groups like mesogenic groups in the molecular structure of one component of thermoplastic polymer blends brings about more arranged and organized structures with considerable orientations and molecular extensions, which resulting in mechanical reinforcements [2,22]. In accord with that, in our previous work, the considerable improvement in the mechanical characteristics of HDPE was obtained due to the increasing of chain orientations of HDPE chains in the matrix, which resulted from the presence of side-chain bonded acrylate units having rigid-rod like molecular structure [23]. The presence of liquid crystalline polymer (LCP) with rigid molecular backbone in the PP matrix raised to improvement in the fibrillation with the use of silica fillers. This dual reinforcements observed in the LCP/SiO2/PP system resulted in the relatively higher yield strength and modulus [24].

On the other hand, the free volume indubitably effects the mechanical properties of the polymers [25,26]. The free-volume and mechanical properties of the polymers possess the anti-correlated relationship due to the fact that the formation of excess amount of holes in matrix damages the microscopic structure of polymers [27]. That is, the molecular motions and mobility of polymer chains in the bulk state are directly related to the presence of the holes, spaces and places where there exist voids and vacancies [28]. As the molecules in matrix are in motion into the holes, the molecules exchange the place with the holes and the more holes can be needed for this movement of the polymer chains [29]. Therefore, the critical free volume must exist in polymer matrix for supplying the optimum molecular motion of the polymeric segments. Burgess et al. reported that the addition of the small caffeine molecule into PET, which results in filling of holes in the matrix, gave rise to the chain motions restrictions and the reduction in the free volume in the film samples compared to neat PET [30]. Kumar et al. studied the relation between free volume and the physico‐mechanical behaviour of polyurethane (PU)/polyacrylonitrile (PAN) polymer networks. The obtained results showed that the maximum tensile strength was achieved with the 70/30 PU/PAN mixture at which the lowest free volume and percent elongation were found. That is, the mechanical characters of the PU/PAN blends affected positively from the decrement in the free volume due to the better interpenetration, higher degree of physical entanglement and hydrogen‐bond formation between PU and PAN [31].

Although the fluorine-containing polymers possess superior properties, there existed any studies in the literature, showing how the mechanical, thermal and microstructural properties of graft copolymers bearing fluorine-containing polymers bonded as side chain were effected from the free volume change. To fill this gap, this current work focused on the examination of the crucial properties of HDPE graft copolymers formed by using the fluorooalkyl acrylate molecule bearing perfluorinated pendant units. By considering stiff molecular backbone, the larger lateral polarity effect, self orientation properties, chain sliding effect, tendency to show smectic phase and ability to increase free volume of this semifluorinated acrylic compound, the main characteristic properties of HDPE were improved via grafting of semifluorinated acrylic compound onto it.