Identification of LDPE Grades Focusing on Specific CH2 Raman Vibration Modes

The possibilities of applications of vibrational spectroscopy techniques (Raman spectroscopy) in the analysis and characterization of polymers are more and more used and accurate. In this paper, our purpose is to characterize Low Density Poly(Ethylene) (LDPE) grades by Raman spectroscopy and in particular with CH2 Raman vibration modes. With temperature measurements, we determine different amorphous and crystalline Raman assignments. From these results and on the basis of the evolution of CH2 bending Raman vibration modes, we develop a phenomenological model in correlation with Differential Scanning Calorimetry and in particular with crystalline lamella thickness determination. 1. Introduction Nowadays, almost 30% of the plastics world production is dedicated to poly(ethylene) (PE) (77 million tons per year) [1]. This polyolefin is considered as a consumer polymer due to its moderate cost of manufacturing and its physical and mechanical properties compatible with various applications in everyday life. Indeed, PE is generally easily processable. It possesses an excellent electric insulation and shock resistance combined with a very good chemical and biological inertia [2]. For each PE grade corresponds of specific applications, presenting different rheological properties. It is then essential to know how to distinguish these products by adapted methods of characterization. Moreover, to be competitive, PE production should be analysed on-line in order to quickly give the main properties of product (Melt Flow Index, additives, flaws, etc.). Raman spectroscopy is an innovative experimental technique of polymer process analysis, which allows doing this. Its use is nowadays growing fast because of advantages. It is a nondestructive method, capable of also giving useful information about the morphology of the polymer. This technique can be perfectly used in industry by means of adapted sensors and devices with more and more reduced dimensions [3]. Raman spectroscopy is used to determine the characteristic transition phases of PE and information about the polymer microstructure [4]. By the use of chemometric tools, for instance, it is possible to identify various Low Density PolyEthylene (LDPE) grades. Polyethylene crystallization can be considered as the transition between fully entangled Gaussian chains and fully extended chain without entanglements ordered in manner to provide crystals. Between two ideal states, “metastable” crystalline and molten states exist [5]. Moreover, it is well known that polyethylene usually crystallizes in an orthorhombic lattice