Micro- and Nano-Structured Interpenetrating Polymer Networks - Sabu Thomas - ebook

Micro- and Nano-Structured Interpenetrating Polymer Networks ebook

Sabu Thomas

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This book examines the current state of the art, new challenges, opportunities, and applications of IPNs. With contributions from experts across the globe, this survey is an outstanding resource reference for anyone involved in the field of polymer materials design for advanced technologies. * Comprehensively summarizes many of the recent technical research accomplishments in the area of micro and nanostructured Interpenetrating Polymer Networks * Discusses various aspects of synthesis, characterization, structure, morphology, modelling, properties, and applications of IPNs * Describes how nano-structured IPNs correlate their multiscale structure to their properties and morphologies * Serves as a one-stop reference resource for important research accomplishments in the area of IPNs and nano-structured polymer systems * Includes chapters from leading researchers in the IPN field from industry, academy, government and private research institutions

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MICRO- AND NANO-STRUCTURED INTERPENETRATING POLYMER NETWORKS

From Design to Applications

 

Edited by

PROF. DR. SABU THOMASDR. DANIEL GRANDEDR. UROŠ CVELBARDR. K.V.S.N. RAJUDR. RAMANUJ NARAYANDR. SELVIN P. THOMASAKHINA H.

 

 

 

 

 

Copyright © 2016 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Names: Thomas, Sabu, editor.Title: Micro- and nano-structured interpenetrating polymer networks : from design to applications / edited by Prof. Dr. Sabu Thomas, Dr. Daniel Grande, Dr. Uroš Cvelbar, Dr. K.V.S.N. Raju, Dr. Ramanuj Narayan, Dr. Selvin P. Thomas, Akhina H.Description: Hoboken : Wiley, 2016. | Includes bibliographical references and index.

Identifiers: LCCN 2015047249 (print) | LCCN 2015050095 (ebook) | ISBN 9781118138175 (hardback) | ISBN 9781119138952 (Adobe PDF) | ISBN 9781119138969 (ePub)Subjects: LCSH: Polymer networks. | Polymers–Industrial applications. | BISAC: TECHNOLOGY & ENGINEERING / Chemical & Biochemical.Classification: LCC QD382.P67 M53 2016 (print) | LCC QD382.P67 (ebook) | DDC 620.1/92–dc23LC record available at http://lccn.loc.gov/2015047249

Cover image courtesy of Getty Images/ Milanares

LIST OF CONTRIBUTORS

Rameshwar Adhikari, Central Department of Chemistry, Tribhuvan University, Kathmandu, Nepal

Akhina H., International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India

Mohammed N. Alghamdi, Mechanical Engineering Technology Department, Yanbu Industrial College, Royal Commission for Yanbu Colleges and Institutes, Yanbu Al-sinaiah, Saudi Arabia

Mladen Andreis, Physical Chemistry Division, Rudjer Bošković Institute, Zagreb, Croatia

S. Anil Kumar, Department of Chemistry, N.S.S. College, Ottapalam, Kerala, India

Prathab Baskar, Materials Modeling and Product Design, Research & Development, Tata Steel, Jamshedpur, Jharkhand, India

Emilio Bucio, Departamento de Química de Radiaciones y Radioquímica, Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Mexico City, México

Guillermina Burillo, Departamento de Química de Radiaciones y Radioquímica, Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, México City, Mexico

Shoubing Chen, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China

Uroš Cvelbar, Department of Surface Engineering, Jozef Stefan Institute, Ljubljana, Slovenia

Branko Dunjić, Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia

Lorena Garcia-Uriostegui, Departamento de Química de Radiaciones y Radioquímica, Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, México City, Mexico

Sudipta Goswami, Chemical Engineering and Technology, Birla Institute of Technology, Mesra, Ranchi, Jharkhand, India

Daniel Grande, Institut de Chimie et des Matériaux Paris-Est (ICMPE), UMR 7182 CNRS-Université Paris-Est Créteil, Thiais, France

Gaohong He, State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, Dalian University of Technology, Dalian, China

Jose James, Research and Post-Graduate Department of Chemistry, St. Joseph’s College, Moolamattom, Kerala, India

Xiaobin Jiang, State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, Dalian University of Technology, Dalian, China

Damir Klepac, Department of Chemistry and Biochemistry, School of Medicine, University of Rijeka, Rijeka, Croatia

Xiangcun Li, State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, Dalian University of Technology, Dalian, China

Sanja Marinović, Department of Catalysis and Chemical Engineering, Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Belgrade, Serbia

Miran Mozetič, Department of Surface Engineering, Jozef Stefan Institute, Ljubljana, Slovenia

Ramanuj Narayan, Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology (IICT), Hyderabad, Telangana, India

Ivanka Popović, Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia

K.V.S.N. Raju, Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology (IICT), Hyderabad, Telangana, India

Chepuri R.K. Rao, Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology (IICT), Hyderabad, Telangana, India

Radhika Raveendran, Division of Biosurface Technology, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Science and Technology, Thiruvananthapuram, Kerala, India

Sa-Ad Riyajan, Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathum Thani, Thailand

Chandra P. Sharma, Division of Biosurface Technology, Biomedical Technology Wing, Sree Chitra Tirunal Institute for Medical Science and Technology, Thiruvananthapuram, Kerala, India

George V. Thomas, Research and Post-Graduate Department of Chemistry, St. Joseph’s College, Moolamattom, Kerala, India

Sabu Thomas, International and Interuniversity Centre for Nanoscience and Nanotechnology and School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Selvin P. Thomas, Yanbu Research Center, Royal Commission for Yanbu Colleges and Institutes, Yanbu Al-sinaiah, Saudi Arabia

Srećko Valić, Department of Chemistry and Biochemistry, School of Medicine, University of Rijeka, Rijeka and Physical Chemistry Division, Rudjer Bošković Institute, Zagreb, Croatia

Qihua Wang, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China

Tingmei Wang, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China

Runcy Wilson, School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Xuemei Wu, State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, Dalian University of Technology, Dalian, China

Wu Xiao, State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, Dalian University of Technology, Dalian, China

Xiaoming Yan, State Key Laboratory of Fine Chemicals, Research and Development Center of Membrane Science and Technology, Dalian University of Technology, Dalian, China

1MICRO- AND NANO-STRUCTURED INTERPENETRATING POLYMER NETWORKS: STATE OF THE ART, NEW CHALLENGES, AND OPPORTUNITIES

Jose James1, George V. Thomas1, Akhina H.2 and Sabu Thomas2,3

1 Research and Post-Graduate Department of Chemistry, St. Joseph’s College, Moolamattom, Kerala, India

2 International and Interuniversity Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, Kerala, India

3 School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

1.1 INTRODUCTION

Polymer mixtures are materials that play a significant role in modern industry. The preparation and properties of multicomponent polymeric systems are of great practical and academic interest. They provide a convenient route for the modifications of properties to meet specific needs. Interpenetrating polymer networks (IPNs) are one of the most rapidly growing areas in polymer material science. The golden history of IPN had begun with its discovery by Aylsworth in 1914 [1].

IUPAC Compendium of chemical terminology defines IPNs as polymers comprising two or more networks that are at least partially interlaced on a molecular scale but not covalently bonded to each other and cannot be separated unless chemical bonds are broken [1, 2]. IPNs are a combination of incompatible polymer networks, at least one of which is synthesized and/or crosslinked in the presence of the other [3, 4] (Figure 1.1).

FIGURE 1.1 Schematic representation of component networks and IPN.

Source: Klempner et al. [3]. Reproduced with permission of the American Chemical Society

Figure 1.2 shows a schematic representation of mechanical blends, graft copolymers, block copolymers, semi-IPNs, and full-IPNs.

FIGURE 1.2 Schematic representation of (a) mechanical blends, (b) graft copolymers, (c) block copolymers, (d) semi-IPNs, and (e) full-IPNs.

Source: Sperling and Mishra [5]. Reproduced with permission of John Wiley & Sons

IPNs are in fact a special class of polymer blends. The two characteristic features of IPNs that distinguishes itself from the other types of multiphase polymer systems are as follows: (1) IPNs swell but do not dissolve in solvents and (2) creep and flow are suppressed in IPNs [5].

While the science of IPNs began with the work of Millar in 1960 [6], the first publication on the subject came through a patent by Aylsworth in 1914 [1]. Since then, IPNs have been the subject of extensive study by investigators looking into the synthesis, morphology, properties, and applications of these materials. Table 1.1 summarizes the history of IPNs and related materials [7, 8].

TABLE 1.1History of IPN

Year

Event

First Investigators

1914

IPN-type structure

J. W. Aylsworth

1927

Graft copolymers

I. Ostromislensky

1951

IPNs

J. J. Staudinger and H. M. Hutchinson

1952

Block copolymers

A. S. Dunn and H. W. Melville

1960

Homo-IPNs

J. R. Millar

1969

Sequential IPNs

L. H. Sperling

1969

Latex IPNs

K. C. Frisch, D. Klempner, and H. L. Frisch

1971

Simultaneous IPNs

L. H. Sperling and R. R. Arnts

1974

IPN nomenclature

L. H. Sperling

1977

Thermoplastic IPNs

S. Davison and W. P. Gergen

1.2 TYPES OF INTERPENETRATING POLYMER NETWORKS

Figure 1.3 classifies the following types of IPNs [1, 9]: semi-IPN, sequential IPN, simultaneous IPN, and full-IPN. They are grouped based on chemical bonding and rearrangement pattern.

FIGURE 1.3 Classifications of IPNs.

Source: Shivashankar, http://www.ijppsjournal.com/. Used under CC-BY-4.0 http://creativecommons.org/licenses/by/4.0/

1.2.1 Full-Interpenetrating Polymer Networks

They comprise two or more polymer networks, which are at least partially interlocked on a molecular scale but not covalently bonded to each other and cannot be separated unless chemical bonds are broken. It can be represented as in Figure 1.4a.

FIGURE 1.4 (a) Full-IPN. (b) Semi-IPN.

Source: Sperling and Mishra [6]. Reproduced with permission of Taylor & Francis

1.2.2 Sequential Interpenetrating Polymer Networks

This type of IPN involves the preparation of a polymer network I followed by the in situ polymerization of monomer II along with crosslinker and activator, which are then swollen into the network I.

1.2.3 Simultaneous Interpenetrating Networks

In simultaneous interpenetrating networks (SINs), the monomers along with the crosslinkers and activators of both networks are mixed. The reactions are carried out simultaneously; however, they are noninterfering types of reactions such as chain and step polymerization reactions.

1.2.4 Latex Interpenetrating Polymer Networks

These IPNs are made in the form of lattices, frequently with a core and shell structure.

1.2.5 Gradient Interpenetrating Polymer Networks

Gradient IPNs are materials in which the overall composition or crosslink density of the material varies from location to location on the macroscopic level. For example, a film can be made with network I predominantly on one surface, network II on the other surface, and a gradient in composition throughout the interior.

1.2.6 Thermoplastic Interpenetrating Polymer Networks

Thermoplastic IPN materials are hybrids between polymer blends and IPNs that involve physical crosslinks rather than chemical crosslinks. Thus, these materials flow at elevated temperatures, similar to the thermoplastic elastomers, and at use temperature, they are crosslinked and behave like IPNs.

1.2.7 Semi-Interpenetrating Polymer Networks

In semi-IPNs, only one component of the assembly is crosslinked leaving the other in linear form. They are also called pseudo-IPNs. It can be depicted as in Figure 1.4b.

Mishra and Sperling [6] investigated semi-IPNs composed of poly(ethylene terephthalate) and castor oil. They found that bond interchange between these two materials played a major role in initial miscibility and morphology. The semi-IPNs displayed much better mechanical properties than the individual component materials did.

1.3 SYNTHESIS OF INTERPENETRATING POLYMER NETWORKS

The IPN synthesis techniques can be summarized as follows.

1.3.1 Sequential Interpenetrating Polymer Networks

This technique involves the sequential addition of selective crosslinkers to a homogenous mixture of two polymers in solution or in melt form [6]. They are in fact synthesized by a two-step process. In the first step, polymerization of first mixture (consisting of monomer, crosslinking agent, and initiator or catalyst) forms a network I. This network is swollen with the second combination of monomer and crosslinking agent and polymerized to form an IPN, that is, the polymer-2 is polymerized and crosslinked in situ in network I [6]. The route can be represented as in Figure 1.5.

FIGURE 1.5 Schematic representation of sequential IPN synthesis.

Source: Sperling [1]. Reproduced with permission of Springer

Nitrile butadiene rubber/poly(ethylene oxide) (NBR/PEO) IPNs prepared by Goujon et al. successfully employed the sequential route for IPN synthesis [10]. Here, IPNs were prepared from NBR and PEO using a two-step process. The NBR network was obtained by dicumyl peroxide crosslinking at high temperature and pressure. A free radical copolymerization of poly(ethylene glycol) methacrylate and dimethacrylate led to the formation of the PEO network within the NBR network. It can be depicted as in Figure 1.6.

FIGURE 1.6 Outline of NBR/PEO IPN synthesis. (a) NBR network synthesis and (b) PEO network synthesis within NBR network.

Source: Goujon et al. [10]. Reprinted with permission of the American Chemical Society

Sequential IPNs based on a nitrile–phenolic blend and poly(alkyl methacrylate) were prepared and characterized by Samui et al. [11]. The IPNs were not fully compatible but exhibited higher tensile strength compared to corresponding nitrile–phenolic blends. The strength increased with the increase in the concentration of poly(alkyl methacrylate).

1.3.2 Simultaneous Interpenetrating Networks

In SINs, a polymer is synthesized (from the monomer) and simultaneously crosslinked within the network of another polymer to give rise to an interpenetrating network [12]. Here, an IPN is formed by polymerizing two different monomers and crosslinking agent pairs together in one step. The key to the success of this process is that the two components must polymerize by reactions that will not interfere with one another. This is often accomplished by polymerizing one network by a condensation reaction, while the other network is formed by a free radical reaction. The route can be represented as in Figure 1.7.

FIGURE 1.7 Schematic representation of simultaneous IPN synthesis.

Source: Sperling et al. [1]. Reproduced with permission of Springer

A notable IPN synthesis by adopting sequential method was reported by M. Ivankovic et al. [13]. Here, IPN consists of methyl methacrylate (MMA) and an organically modified silicon alkoxide, 3-glycidyl oxy propyltrimethoxysilane (GLYMO), with varying MMA/GLYMO molar ratios.

SINs were first reported by Frisch by blending a urethane prepolymer with a low-molecular-weight epoxy resin [7]. In SINs, the relative rates of formation of each network would be the controlling factor in determining the morphology. It was also observed that the morphology of IPNs was slightly finer than the corresponding pseudo-IPN [8].

Classical works on IPN include damping applications [14], photodiodes from IPN [15], IPN membranes [16], complex-forming IPNs [17], fire-retarding IPNs [18], conducting IPNs [19], encapsulating IPNs, conducting hydrogel IPNs [20], elastomeric IPNs [21], and polymer solar cell based on IPNs [22].

1.4 CHARACTERIZATION OF INTERPENETRATING POLYMER NETWORKS

Modern instrumental techniques are used to characterize the interpenetration and ultimate properties of IPNs. Various methods are there to determine the morphology, thermal properties, physical properties, and other characteristics of IPNs. The interpenetration in IPN may be identified by:

Comparing the shift of dynamic glass transition temperatures of IPNs with their homopolymers.

Comparing the morphologies (micro- or nanoscale) and distribution of phase domains, shape, and sizes.

The existence of interpenetration is generally better judged through the combination of aforementioned inferences [23].

1.4.1 Morphology

The morphology of IPNs depends on the method of synthesis, on the compatibility of the polymer systems employed, and on the relative rates of formation. In sequential IPNs, the network first formed is most likely to be the continuous network. Its crosslink density is the controlling factor in determining the morphology of the system of each network [24]. The morphological properties of IPNs are best characterized by using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

The micrographs obtained using the SEM and TEM appears to be useful in detecting the interpenetration of phase domains, shape, and structures of the order of magnitude of a micron to a tenth of a micron and the degree of mixing [25].

TEM is another powerful technique for the elucidation of IPN architecture.TEM analysis proves the fine distribution of PEO phase in continuous NBR phase for the IPN of the PEO–NBR system [10] (Figure 1.8).

FIGURE 1.8 TEM pictures of NBR/PEO (50% PEGDM) IPN.

Source: Goujon et al. [10]. Reprinted with permission of the American Chemical Society

Here, the black domains correspond to NBR-rich phases, and the white domains represent the PEO-rich phases. Since the TEM pictures are grayish, the boundary between the dark and clear gray regions is not obvious. This reinforces the fact that the IPN architecture is a very finely distributed PEO phase in a continuous NBR matrix.

A recent work on magnetically doped multistimuli-responsive hydrogel microspheres with IPN structure reported by Ahmad et al. [26] successfully employed SEM and TEM images for morphology characterization. In this study, it is found that the presence of Fe3O4 nanoparticles on the surface of P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres is clearly observed from microscope images. Both TEM and SEM images suggest that the deposition of Fe3O4 nanoparticles on the surface of P (NIPAM-MBAM)/P (MAA-EGDM) IPN hydrogel microspheres most likely improved the stability during sample preparation as the microspheres remained mostly spherical (Figures 1.9 and 1.10).

FIGURE 1.9 TEM images of (a) P(NIPAM-MBAM) hydrogel microspheres, (b) P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres, (c) Fe3O4 particles, and (d) P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres.

Source: Ahmad et al. [26]. Reprinted with permission of Elsevier

FIGURE 1.10 SEM image of washed P (NIPAM-MBAM)/P (MAA-EGDM)/Fe3O4IPN hydrogel microspheres.

Source: Ahmad et al. [26]. Reprinted with permission of Elsevier

IPN based on polyisoprene and PMMA by John et al. [27] also employed SEM images as a powerful tool for characterization. SEM images of IPN (poly isoprene–PMMA IPN) at different compositions are given in the succeeding text. These are SEM images of IPNs with same crosslink density and varying compositions.

Figure 1.11a shows the sample with 20 wt% PMMA. It has a sea-island morphology. Figure 1.11b shows the sample with 35% PMMA. It shows a discrete state with larger domain dimension. In Figure 1.11c, the 50% PMMA composition shows a dual-phase morphology with polyisoprene as the matrix. The sample with 60% PMMA composition shown in Figure 1.11d shows a continuous phase morphology.

FIGURE 1.11 SEM images of PI-PMMA (a) with 20 wt% PMMA, (b) with 35% PMMA, (c) with 50% PMMA, and (d) with 60% PMMA.

Source: John et al. [28]. Reproduced with permission of Elsevier

An unusual semi-IPN nanoencapsulating layer composed of thermally cured polyimide (PI) and polyvinyl pyrrolidone (PVP) with lithium cathode materials (here LiCoO2 (LCO)) reported by Kim et al. utilized the potential application of SEM and TEM images for morphology characterization [29]. The surface morphology of PI/PVP-LCO was characterized with a focus on coverage area of the LCO surface. Figure 1.12a shows that the PI/PVP-LCO has highly continuous and conformal polymeric layers on the LCO surface. The highly continuous conformal morphology of the PI/PVP nanoencapsulating layer was further elucidated by conducting TEM studies (Figure 1.12b).

FIGURE 1.12 Structural characterization of PI/PVP-LCO. (a) FE-SEM photograph; (b) TEM photographs before/after selective etching of PVP phase.

Source: Kim et al. [29]. Reproduced with permission of Nature Publishing Group

Atomic force microscopy (AFM) analysis also provides valuable information on the IPN morphology. It helps to differentiate continuous phase and dispersed phase in IPN. AFM is also used to identify the domain size of the components in IPN. In the characterization of polyisobutene (PIB)–PMMA IPN, AFM was used successfully to elucidate the morphology as shown in Figure 1.13a and b [30].

FIGURE 1.13 (a) AFM images of PIB/PMMA (60/40) IPN. (b) AFM images of PIB/PMMA (30/70) IPN.

Source: Vancaeyzeele et al. [30], figures 6, 7. Reprinted with permission of Elsevier

Confocal laser-scanning microscopy (CLSM) also provides some useful insight into IPN morphology [31]. It helps to identify the hydrophilic and hydrophobic regions of hydrogel type IPN. X-ray scattering experiments are widely used for enlightening the morphology of IPN [32]. It can be used for the visualization of interpenetrating channels in IPN. Energy-dispersive X-ray analysis (EDAX) explains the distribution of dispersed phase in the IPN matrix [33]. SEM pictures and EDAX analysis give immediate information about the repartition of polyethylenedioxythiophene (PEDOT) within the NBR matrix. The scanning electron micrograph of a cross section of NBR/PEDOT semi-IPN obtained at 10 kV is shown in Figure 1.14a.

FIGURE 1.14 (a) SEM images of NBR-PEDOT network. (b) EDAX analysis showing the repartition of sulfur, (c) that of oxygen in the NBR-PEDOT semi-IPN.

Source: Francke et al. [33]. Reprinted with permission of Elsevier

As expected, this picture clearly shows the formation of a heterogeneous system: the two different phases observed correspond to pure NBR (phase 1) and PEDOT/NBR (phase 2). We can deduce that PEDOT interpenetrates within the NBR matrix. This observation corroborates the fact that PEDOT cannot be removed from the surface of the matrix by scratching. From the SEM picture, the depth of interpenetration can be estimated around 35 µm. Moreover, the distribution of the characteristic elements S and O of PEDOT in the cross-sectional area observed in the EDAX mapping (Figure 1.14b and c) clearly indicated that PEDOT was present inside the NBR in its doping state as expected.

Laser scanning confocal microscopy (LSCM) is a very good technique to provide the morphology of IPN as a function of depth [34].

1.4.2 Thermal Properties

The collective observations from electron microscopy and thermal measurements provide the real morphology of an IPN. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), differential thermal analysis (DTA), and so on have been extensively used to study the thermal properties of IPN [35].

DSC shows one or two Tg values (shifted or unshifted) based on the extent of interaction between the components. The Tgs often lie at or between the Tgs of the pure networks. The appearance of single Tg indicates the formation of an IPN, and in this case, one can rule out the formation of any macro-/micro-phase separation. The slight shifting and broadening of the peaks indicate a moderate degree of molecular mixing [35]. In many cases, two distinct Tgs corresponding to those individual component networks have been observed. Such behavior has been reported by Sperling and others [36]. Merging of Tgs or an inward shift can be considered as evidence for the interpenetration of phases in IPN [37] (