Enhancement of Adhesion Characteristics of Low-Density Polyethylene Using Atmospheric Plasma Initiated-Grafting of Polyethylene Glycol

Article Enhancement of Adhesion Characteristics of Low-Density Polyethylene Using Atmospheric Plasma Initiated-Grafting of Polyethylene Glycol

Taghreed Abdulhameed Al-Gunaid 1,2, Igor Krupa 1, Mabrouk Ouederni 3, Senthil Kumar Krishnamoorthy 3 and Anton Popelka 1,*

1 Center for Advanced Materials, Qatar University, P.O. Box 2713 Doha, Qatar; [email protected] (T.A.A.-G.); [email protected] (I.K.) 2 Materials Science and Technology Program, College of Arts and Science, Qatar University, P.O. Box 2713 Doha, Qatar 3 Product Development & Innovation, Qatar Petrochemical Company (QAPCO), P.O. Box 756 Doha, Qatar; [email protected] (M.O.); [email protected] (S.K.K.) * Correspondence: [email protected]; Tel.: +974-4403-5676

Abstract: The low-density polyethylene/aluminum (LDPE/Al) joint in Tetra Pak provides stability and strength to food packaging, ensures protection against outside moisture, and maintains the nutritional values and flavors of food without the need for additives in the food products. However, a poor adhesion of LDPE to Al, due to its non-polar surface, is a limiting factor and extra polymeric interlayers or surface treatment is required. Plasma-assisted grafting of the LDPE surface with different molecular weight compounds of polyethylene glycol (PEG) was used to improve LDPE/Al Citation: Al-Gunaid, T.A.; Krupa, I.; adhesion. It was found that this surface modification contributed to significantly improve the Ouederni, M.; Krishnamoorthy, S.K.; wettability of the LDPE surface, as was confirmed by contact angle measurements. The chemical Popelka, A. Enhancement of Adhesion Characteristics of composition changes after plasma treatment and modification process were observed by X-ray Low-Density Polyethylene Using photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). A surface Atmospheric Plasma morphology was analyzed by scanning electron microscopy (SEM) and atomic force microscopy Initiated-Grafting of Polyethylene (AFM). Adhesion characteristics of LDPE/Al adhesive joints were analyzed by the peel tests. The Glycol. Polymers 2021, 13, 1309. most significant adhesion improvement of the PEG modified LDPE surface was achieved using https://doi.org/10.3390/ 10.0 wt.% aqueous (6000 M) PEG solution, while the peel resistance increased by approximately polym13081309 54 times in comparison with untreated LDPE.

Academic Editor: Choon-Sang Park Keywords: polyethylene; surface modiﬁcation; corona discharge; polyethylene glycol; adhesion

Received: 28 March 2021 Accepted: 12 April 2021 Published: 16 April 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Polyolefins are the largest class of synthetic thermoplastic polymers that are employed with regard to jurisdictional claims in in a wide variety of applications nowadays, particularly food packaging, industrial appli- published maps and institutional affil- cations, consumable products, structural plastics, and medical applications [1,2]. This is iations. because these polymers are distinguished by light weight, excellent chemical and physical properties, cost effectiveness, as well as ease of processing [3,4]. Polyolefins are manu- factured by cracking the petrochemical sources such as crude oil and natural gas [5,6]. Polyethylene (PE) polymers including low-density polyethylene (LDPE) and high-density polyethylene (HDPE) are the most famous polyolefins commonly employed in the food Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. packaging industry, since they are easily heat sealable, can be fabricated into rigid films, This article is an open access article with a good barrier against moisture and water vapor [7,8]. In spite of such features, their distributed under the terms and poor surface properties, including adhesion, wettability, and cytocompatibility, impede conditions of the Creative Commons their integration with other materials to form multi-layered laminates. In fact, PE materials Attribution (CC BY) license (https:// have low surface reactivity and hydrophobic nature due to the lack of functional groups creativecommons.org/licenses/by/ and low proportion of polar regions on their surfaces, and therefore incomplete adhesion 4.0/). with other materials [9,10]. Therefore, several surface modification methods have been

Polymers 2021, 13, 1309. https://doi.org/10.3390/polym13081309 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 1309 2 of 19

developed in recent decades. All these methods increase the surface energy of the polymer films, resulting in better wettability and thus higher bond strength [11]. The surface modification methods are basically classified into three sections: physical modification based on plasma technologies and flame treatment, chemical modification via surface functionalization, and mechanical abrasion [12–14]. However, previous studies found that mechanical abrasion could lead to significant damage of the treated surfaces [15]. The flame treatment is difficult to control, and bonding must be carried out shortly after exposure to flame [3]. Therefore, the use of plasma techniques and chemical methods are preferable in the surface treatment of polyolefins. In the industrial scale, corona plasma discharge is a preferred cold plasma technique in surface modification of PE. It promotes surface activation, which leads to enhanced wetting and adhesion characteristics for applications related to adhesive bonding and printing [16,17]. Corona discharge is characterized by fast operation and completion (few seconds for treatment), cost effectiveness, easily adaptable to in-line operations, and envi- ronmentally friendly without the need to use aggressive chemicals during operation [3,18]. Corona discharge occurs when ambient air molecules are ionized at atmospheric pressure into charged particles such as electrons and ions. A high electric potential difference is formed between two asymmetric conductive electrodes (high-potential electrode and a grounded electrode) separated by a gap containing air. This creates a large electric field that accelerates the charged particles toward the polymer surface, which leads to the incorporation of reactive functional groups on the surface such as carbonyl, hydroxyl, hydroperoxides, aldehydes, ethers, esters, etc. The formed functional groups increase the polar part of surface energy and thus also the overall surface energy. Consequently, the surface is oxidized, its roughness and wettability increase, and finally its adhesion with other materials remarkably improves [11,17,19,20]. Chemical modification of the polymeric surfaces using graft polymerization plays a vital role in biomedical, environmental, and industrial applications [21,22], since it con- tributes to positive changes in the physical and chemical properties, morphology, and biocompatibility of the polymer [23–25]. Polyethylene glycol (PEG) is a versatile hydrophilic polyether that is immobilized onto the polymer surfaces using various techniques as physical adsorption, graft polymerization, covalent grafting, blending, etc. [26–29]. It is synthesized via chain-growth ring-opening polymerization of ethylene oxide in the presence of methanol or water as an initiator [30]. PEG is available as linear or branched chain polymers with an oxyethylene (-O-CH2-CH2-) repeating units that bonded with hydroxyl groups on either side of its chain [26,31,32]. The molecular weight of PEG plays a considerable role in specifying its properties. PEG is known as polyethylene oxide (PEO) when it is present in the form of a solid crystalline powder with molecular weight (M) greater than 20,000 g/mol, while PEG exists as viscous liquid (M < 1000 g/mol) or wax-like solid form (M: 1000–20000 g/mol) [26,33]. PEG and PEO compounds are soluble in both aqueous and organic solvents [34,35]. Recently, many studies focused on the development of the surface modification of hydrophobic polymers via graft polymerization with PEGs. Liu et al. worked on surface modification of polyester urethane (SPEU) films with different molecular weights of PEG compounds, Mn = 1200, 2400, and 4000 g/mol, for biomedical purposes. The SPEU surface was modified by grafting PEG on its surface, since PEG can effectively prevent protein adsorption and platelet adhesion due to its low interfacial free energy with water, unique solution properties, hydrophilicity, high chain mobility, and steric stabilization effect. The results showed that with increasing the molecular weight of PEG, there was a significant decrease in the water contact angle on PEG-g-SPEU, which indicated an increase in the surface energy and polarity, and thus strongly hydrophilic SPEU surface. Also, this can be attributed to high grafting density of PEG on the SPEU-PEG surface [36]. Adib and Raisi studied the surface modification of polyether sulfone membrane by grafting with hyperbranched PEG in combination with corona air plasma with the aim of enhancing anti-fouling properties. This led to improvement of the anti-fouling Polymers 2021, 13, 1309 3 of 19

property and oil–water permeability of all modified membranes without any significant changes in oil rejection [37]. In this work, the plasma imitated grafting of PEG/PEO on the LDPE surfaces were employed to improve adhesion characteristics without changing bulk properties. In fact, this research shed light on studying the effect of the difference in the molecular weights of PEG/PEO as well as the concentration of the PEG/PEO-based prepared aqueous solutions in the enhancement of the surface properties of LDPE in order to achieve higher interfacial adhesion of modified LDPE with Al to form LDPE/Al adhesive laminates that are commonly used for food packaging and processing applications (e.g., Tetra Pak containers). However, evident changes in surface characteristics of LDPE specimens after PEG/PEO grafting were demonstrated using several analysis techniques. These include surface hydrophilicity or wettability, chemical compositions of the surface, as well as surface roughness and morphology. Last but not least, the adhesion strength between modified LDPE and Al was improved. The adhesion characteristics were analyzed using two different methods, namely peel test at constant 90◦ angle, and work of adhesion calculations based on contact angle measurements.

2. Materials and Methods 2.1. Materials Low-density polyethylene (LDPE) from Qatar Petrochemical Company (QAPCO, Mesaieed, Qatar) with code number: EC01-049 was used in this research. The used LDPE in granular form were hot-pressed into a thin transparent sheet using a hydraulic press machine (Carver, Wabash, IN, USA). Some characteristics of the LDPE are summarized in Table1. LDPE sheets were bonded with aluminum (Al) foil (GLAD ®, Qingdao, Shandong, China) to produce a coherent adhesive joint (LDPE/Al laminates), achieving the main purpose of this work.

Table 1. The properties/technical information of low-density polyethylene (LDPE) (EC01-049, QAPCO).

LDPE Properties Description Density at temperature 23 ◦C 0.918 g/cm3 (ASTM D-1505) Melt ﬂow index 8.0 g/10 min, 190 ◦C/2.16 kg (ASTM D-1238) Crystalline melting point 105 ◦C Recommended uses Extrusion coating at high speed

In addition, acetone (min.99.8% assay by G.C. method, Scharlab S.L., Barcelona, Spain) was used to remove any impurities or contaminants from the LDPE and Al surfaces prior to applying the surface treatment. For the wettability investigation of LDPE surfaces, ultra- pure water (Purity ≥ 99%, water purification system Direct-Q®, Millipore Corporation, Molsheim, France), formamide (Purity > 98%, FLUKA™, Merelbeke, Belgium), ethylene glycol (Purity ≥ 98%, FLUKA™, Morris Plains, NJ, Belgium) were used as testing liquids with different surface tension to determine the changes in surface total surface free energy and its components of the LDPE samples based on contact angle measurements. For surface modification of LDPE surfaces via grafting, PEG compounds with different molecular weights (M): 1000 g/mol (Fluka Chemika, Buchs, Switzerland), and 6000 g/mol (Merck KGaA, Darmstadt, Germany), as well as PEO with M = 300,000 g/mol (Sigma-Aldrich corporation, MO, St. Louis, USA) were used to increase the adhesion characteristics of LDPE surfaces. These compounds were dissolved into distilled water to prepare aqueous solutions at specific concentrations.

2.2. Preparation of LDPE Thin Sheets and LDPE-Al Laminate The LDPE granulates were converted into coherent thin sheets using a hydraulic mounting press machine (Carver, Wabash, IN, USA). Ten grams of LDPE granules were placed between two transparent polyester sheets inside two highly polished stainless-steel plates, with a concern that granules were positioned adjacent to each other and on one level. Polymers 2021, 13, x FOR PEER REVIEW 4 of 19

Polymers 2021, 13, 1309 4 of 19 placed between two transparent polyester sheets inside two highly polished stainless- steel plates, with a concern that granules were positioned adjacent to each other and on Afterone level. that, all After the previouslythat, all the prepared previously were prepared entered betweenwere entered the upper between and lowerthe upper molding and plateslower of molding the hydraulic plates of press the machine.hydraulic LDPEpress machine. granules wereLDPE heated granules up were into a heated temperature up into slightlya temperature higher thanslightly the meltinghigher than temperature the melt (160ing temperature◦C). Once the (160 desired °C). temperatureOnce the desired was reached,temperature a one-ton was reached, load was a applied one-ton into load the was LDPE applied granules into forthe two LDPE minutes, granules to convert for two theseminutes, granules to convert into athese thin sheetgranules under into the a th inﬂuencein sheet under of applied the influence temperature of applied and force. tem- Finally,perature the and prepared force. LDPEFinally, sheet the wasprepared cooled LDPE down sheet gradually was untilcooled room down temperature. gradually Theuntil thicknessroom temperature. of the LDPE The sheets thickness was foundof the toLDPE be approximately sheets was found 290 µtom be and approximately LDPE samples 290 wereµm and cleaned LDPE by samples acetone inwere order cleaned to remove by acetone all undesirable in order contaminantsto remove all fromundesirable the surface con- priortaminants to every from post the treatment/modiﬁcation surface prior to every process.post treatment/modification Furthermore, the LDPE/Al process. adhesive Further- jointsmore, were the LDPE/Al fabricated adhesive by lamination joints processwere fabricated using mounting by lamination hot press process machine using with mounting almost thehot same press steps machine as LDPE with sheet almost preparation the same (twosteps tons as LDPE compression sheet preparation molding for (two 2 min tons at com- 160 ◦ pressionC, then cooling molding to for room 2 min temperature). at 160 °C, then cooling to room temperature).

2.3.2.3. Surface Surface Modiﬁcation Modification of of LDPE LDPE Surface Surface Using Using Corona Corona Discharge Discharge TheThe LDPE LDPE surface surface was was treated treated using using corona corona plasma plasma discharge discharge in order in order to introduction to introduc- oftion polar of polar functional function groups.al groups. A laboratoryA laboratory scale scale corona corona plasma plasma system system (CVE-L, (CVE-L, Softal, Softal, Hamburg,Hamburg, Germany) Germany) (Figure (Figure1 )1) was was employed employed for for surface surface treatment treatment of of LDPE LDPE foils foils under under atmospheric pressure using 300 W of nominal power and 17.20 kHz of frequency. The atmospheric pressure using 300 W of nominal power and 17.20 kHz of frequency. The plasma treatment process of LDPE was optimized by varying treatment time from 1 to 7 s, plasma treatment process of LDPE was optimized by varying treatment time from 1 to 7 while the optimal treatment time was achieved using 5 s, which was associated with the s, while the optimal treatment time was achieved using 5 s, which was associated with the best achieved wettability. This system contains a catalytic ozone removal system ensuring best achieved wettability. This system contains a catalytic ozone removal system ensuring a safe working environment. Applied high potential between the biased and grounded a safe working environment. Applied high potential between the biased and grounded electrode (1.5 mm gap distance) using ambient air was responsible for homogeneous electrode (1.5 mm gap distance) using ambient air was responsible for homogeneous sur- surfaces treatment of LDPE. The LDPE samples were treated from both sides. faces treatment of LDPE. The LDPE samples were treated from both sides.

FigureFigure 1. 1.Corona Corona plasma plasma discharge discharge system system (Softal, (Softal, Hamburg, Hamburg, Germany) Germany). .

2.4.2.4. Grafting Grafting PEG/PEO PEG/PEO onto onto LDPE LDPE TheThe corona-treated corona-treated LDPE LDPE specimens specimens were were completely completely immersed immersed into into specific specific concen- concen- trationstrations of of PEG- PEG- or or PEO-based PEO-based aqueous aqueous solutions solutions at at room room temperature temperature for for 24 24 h h (Figure (Figure2 ).2). AfterAfter the the modification modification process, process, the the LDPE LDPE specimen specimen was was thoroughly thoroughly rinsed rinsed with with distilled distilled waterwater immediately immediately after after extraction extraction from from the the solution, solution, to to remove remove unreacted unreacted species species from from thethe LDPE LDPE surface. surface. Thereafter, Thereafter, it it was was left left to to totally totally dry dry at at room room temperature temperature prior prior to to other other characterizationscharacterizations andand laminationlamination withwith Al. Al. SixSix differentdifferent aqueous aqueous solutions solutions were were used used in in thisthis work, work, regarding regarding two two different different concentrations concentrations per per each each PEG/PEO PEG/PEO molecular molecular weight, weight, toto investigate investigate the the influence influence of of changing changing the the molecular molecular weight weight of of PEG/PEO PEG/PEO chains, chains, and and concentrationconcentrationof of prepared prepared PEG/PEO PEG/PEO aqueousaqueous solutionssolutions onon the the surface surface characteristics characteristics of of LDPE.LDPE. TheseThese concentrationsconcentrations were as follows: 1.5 1.5 wt.%, wt.%, and and 10.0 10.0 wt.% wt.% for for 1000 1000 M MPEG, PEG, 1.5 1.5wt.% wt.% and and 10.0 10.0 wt.% wt.% for for 6000 6000 M M PEG, PEG, and and 1.5 1.5 wt.% and 5.05.0 wt.%wt.% (maximal(maximal solubilitysolubility in in water)water) for for 300,000 300,000 M M PEO. PEO. Polymers 2021, 13, 1309 5 of 19 Polymers 2021, 13, x FOR PEER REVIEW 5 of 19

OOH OOCH2 CH2 OH C C n Plasma PEG/PEO CH2 CH2 CH CH2 CH CH2 n n n LDPE

FigureFigure 2.2. Scheme of proposed grafting grafting mechanism mechanism of of polyethylene polyethylene glycol glycol or or polyethylene polyethylene oxide oxide (PEG/PEO)(PEG/PEO) ontoonto corona-treatedcorona-treated LDPE.LDPE. 2.5. Grafting Efficiency (GE) Evaluation 2.5. Grafting Efficiency (GE) Evaluation Grafting efficiency (GE) in the grafting process was defined as the percentage of the Grafting efficiency (GE) in the grafting process was defined as the percentage of the amount of the grafted monomer, which is linked into the polymer backbone to the total amount of the grafted monomer, which is linked into the polymer backbone to the total amount of the free polymer. GE values of the PEG/PEO grafted on LDPE (PEG/PEO-g- amount of the free polymer. GE values of the PEG/PEO grafted on LDPE (PEG/PEO-g- LDPE) specimens were calculated gravimetrically using Equation (1) LDPE) specimens were calculated gravimetrically using Equation (1) m − m GE [%GE] = [%] = 1 0 ×× 100%100% (1) (1) m0 where m is the mass of the LDPE sample before grafting, m is the mass of the LDPE where m is the mass of the LDPE sample before grafting, m is the mass of the LDPE sample after0 grafting with PEG/PEO. 1 sample after grafting with PEG/PEO.

2.6.2.6. DeterminationDetermination ofof SurfaceSurface WettabilityWettability TheThe changeschanges inin surfacesurface wettabilitywettability afterafter plasmaplasma treatmenttreatment andand modiﬁcationmodification ofof thethe LDPELDPE samples samples were were investigated investigated by by measuring measuring the the contact contact angle angle of selected of selected testing testing liquids. liq- Threeuids. testingThree testing liquids liquids with different with different surface tensionssurface tensions and polarities and polarities were employed were employed in sessile dropin sessile contact drop angle contact measurements, angle measurements, such as water, such formamide, as water, andformamide, ethylene and glycol ethylene (see Table gly- 2col). Contact (see Table angle 2). measuringContact angle system measuring OCA 35 system (Dataphysics, OCA 35 Filderstadt, (Dataphysics, Germany) Filderstadt, was used Ger- formany) this purpose.was used This for systemthis purpose. was connected This syst toem an opticalwas connected video-base to imaging an optical system video-base linked toimaging high-resolution system linked USB camera to high-resolution (up to 2200 images/s). USB camera According (up to 2200 to the images/s). sessile drop According method, 3toµ Lthe volume sessiledroplet drop method, of each 3 testing µL volume liquid drople was deposedt of each softly testing with liquid constant was deposed dosing rate softly of 2withµL/s constant on the LDPE dosing samples rate of with 2 µL/s the on dimensions the LDPE of samples8 cm length with× the2 cmdimensions width. Then, of 8 thecm contactlength angle× 2 cm was width. measured Then, afterthe contact 3 s to ensure angle thatwas themeasured liquid droplet after 3 spreadss to ensure evenly that and the completelyliquid droplet over spreads the surface, evenly while and thermodynamic completely over equilibrium the surface, was while achieved. thermodynamic At least ﬁve separateequilibrium readings was forachieved. each testing At least liquid five were separate taken readings to obtain for one each representative testing liquid average were contacttaken to angle obtain value one thatrepresentative was subsequently average used contact in the angle calculation value that of solid/liquidwas subsequently interfacial used tensionin the calculation based on the of Owens-Wendt-Rabel-Kaelble solid/liquid interfacial tension method based (OWRK-model). on the Owens-Wendt-Rabel- OWRK-model expressesKaelble method the interfacial (OWRK-model). interactions OWRK-model along the solid expresses and liquid the molecules interfacial (γsl )interactions in term of p threealong components, the solid and the liquid total molecules surface free (𝛾 energy) in term (γ )of and three its components, components: the polar total (γ surface) and dispersivefree energy (γ (d𝛾)) components,and its components: by Equation polar (2). (𝛾) and dispersive (𝛾) components, by Equa- tion (2). ! q r d d p p γ = γs + γ − 2 γ .γ + γ .γ (2) sl l s l s l 𝛾 = 𝛾 + 𝛾 − 2𝛾 .𝛾 + 𝛾 .𝛾 (2)

2.7. Determination of the Adhesion Strength of LDPE/Al Laminate 2.7. DeterminationThe 90◦ peel testof the measurements Adhesion Strength were of employedLDPE/Al Laminate for the evaluation of the adhesion characteristicsThe 90° peel between test measurements LDPE and Al components were employed that formfor the together evaluation a coherent of the laminate.adhesion Peelcharacteristics tester LF-Plus between (Lloyd LDPE Instruments, and Al components West Sussex, that UK) form based together on ASTM a coherent D6862 standardlaminate. testPeel method tester LF-Plus was employed (Lloyd Instruments, in the adhesion West Sussex, strength UK) measurements. based on ASTM This D6862 system standard was connectedtest method to was NEXYGENPlus employed in testingthe adhesion software, strength which measurements. allows entering This the system basic data was andcon- experimentalnected to NEXYGENPlus conditions that testing ﬁt the software, test type, whic as wellh allows as the entering results displayed the basic asdata numerical and ex- valuesperimental and representativeconditions that graphs. fit the test Laminated type, as LDPE/Al well as the strips results with displayed dimensions as numerical approxi- matelyvalues ofand 8 cmrepresentative height and 2graphs. cm width Laminate were attachedd LDPE/Al tightly strips on with an acrylic dimensions two sided approxi- tape (3mately M 4910 of k,8 VHBTM)cm height prior and 2 to cm starting width the were test. attached The peel tightly strength on an (the acrylic force pertwo unit sided width tape of(3 theM 4910 laminate) k, VHBTM) was measured prior to starting under dynamic the test. The conditions: peel strength 1-kN load(the force cell was per appliedunit width at Polymers 2021, 13, x FOR PEER REVIEW 6 of 19 Polymers 2021, 13, 1309 6 of 19

of the laminate) was measured under dynamic conditions: 1-kN load cell was applied at 90◦ angle peeling on the specimen, operated at slow speed rate (v = 10 mm/min) to ensure 90° angle peeling on the specimen, operated at slow speed rate (v = 10 mm/min) to ensure the applied peeling force is evenly distributed over the surface, and the test time was set the applied peeling force is evenly distributed over the surface, and the test time was set at 360 s to ensure that LDPE ultra-thin layer was completely separated from the Al foil. at 360 s to ensure that LDPE ultra-thin layer was completely separated from the Al foil. The peel resistance (peel force per width) was evaluated from a 10–50 mm distance of the The peel resistance (peel force per width) was evaluated from a 10–50 mm distance of the LDPE/Al laminate. Following the Standard Test Method for 90 Degree Peel Resistance LDPE/Al laminate. Following the Standard Test Method for 90 Degree Peel Resistance of of Adhesives (ASTM D6862); 4–5 separate readings of LDPE-Al adhesives were taken to Adhesives (ASTM D6862); 4–5 separate readings of LDPE-Al adhesives were taken to ac- acquire one average value of the peel resistance, and subsequently compared with the workquire ofone adhesion average computedvalue of the from peel contact resistan anglece, and measurements. subsequently compared with the work of adhesion computed from contact angle measurements. Table 2. Surface free energy and its components: dispersion and polarity of testing liquids at 23 ◦C. Table 2. Surface free energy and its components: dispersion and polarity of testing liquids at 23 °C. Surface Energy, Dispersion, Polarity, Testing Liquid p γ (mN/m) γd(mN/m) γ (mN/m) Surfacel Energy, 𝜸 Dispersion,l 𝜸𝒅 l Testing Liquid 𝒍 𝒍 Polarity, 𝜸𝒑 (mN/m) Water(mN/m) 72.1 (mN/m) 19.9 52.2𝒍 Formamide 56.9 23.5 33.4 EthyleneWater glycol 72.1 48.0 19.929.0 19.052.2 Formamide 56.9 23.5 33.4 2.8.Ethylene Calculation glycol of the Work of Adhesion 48.0 29.0 19.0 The work of adhesion (W ) for a solid–solid combination is defined as the reversible 2.8. Calculation of the Work of Adhesion12 thermodynamic work (energy change per unit area) that is required to separate two adherentThe work materials of adhesion to form a(W laminate) for a fromsolid–solid the equilibrium combination state is into defined a separation as the reversible distance thermodynamic work (energy change per unit area) that is required to separate two ad- of infinity (Figure3)[ 38]. In this work, quantities W12 of untreated, plasma treated and modifiedherent materials LDPE in to the form LDPE/Al a laminate laminate from were the equilibrium calculated from state contact into a angle separation measurements distance dependingof infinity (Figure on the polarity 3) [38]. andIn this dispersion work, quantities values of the W12 surface of untreated, energy byplasma the Young–Dupr treated andé equationmodified (EquationLDPE in the (3)), LDPE/Al as follows laminate [39]: were calculated from contact angle measurements depending on the polarity and dispersion values of the surface energy by the Young–Dupré equation (EquationW (3)),12 = asγ follows1 + γ2 − [39]:γ12 (3) 𝑊 = 𝛾 +𝛾 −𝛾 (3) where γ1 is the surface energy of LDPE, γ2 is the surface energy of Al, γ12 is the interfa-

cialwhere energy 𝛾 is between the surface LDPE energy and of Al LDPE, (solid–solid 𝛾 is the interface) surface energy and can of be Al, determined 𝛾 is the interfa- by the followingcial energy equation. between LDPE and Al (solid–solid interface) and can be determined by the fol-

lowing equation. 1 1 P P 2 d d 2 γ = γ + γ − 2 γ × γ − 2 γ × γ (4) 12 1 2 1 2 1 2 𝛾 = 𝛾 + 𝛾 − 2𝛾 × 𝛾 − 2𝛾 × 𝛾 (4)

by substituting Equation Equation (4) (4) into into Equation Equation (3); (3); the the work work of of adhesion adhesion (W (W)12 is) isobtained obtained as as follows: follows: 1 1 P P 2 d d 2 = [ × + × ] W12 2 γ1 γ2 γ1 γ2 (5) W = 2𝛾 × 𝛾 + 𝛾 × 𝛾 (5) 0 0 where subscripts ‘1′ and ‘2′ refer to LDPE and Al respectively;respectively; the superscript ‘d’ repre- sents to the non-polar/dispersive contribution; and the superscript ‘p’ refers to the polar sents to the non-polar/dispersive contribution; and the superscript ‘p’ refers to the polar contribution to the surface free energy. contribution to the surface free energy.

Figure 3. Illustrative scheme of work of adhesion between LDPE substratesubstrate andand AlAl ﬁlm.film.

Polymers 2021, 13, 1309 7 of 19

2.9. Surface Morphology Analysis The changes in two-dimensional surface morphology and roughness of the LDPE samples before and after surface modification were investigated by scanning electron microscopy (SEM) (Nova NanoSEM 450, Hillsboro, OR, USA). The LDPE specimens were observed at a high magnification (20,000×) and at high spatial resolution in order to achieve a high quality of the observed images. The working distance (WD) between the source of electrons and the exposed surface of the sample was set within the range of 4.6–5.1 mm. Furthermore, the SEM system was operated with moderate acceleration voltage equal to 5.0 kV. LDPE surfaces were coated by a thin layer (few angstroms thickness) of a gold (Au) to ensure higher resolution of captured SEM images, as well as to prevent charging of the surface and to promote the emission of secondary electrons [40]. The three-dimensional changes in the surface topography and roughness of the LDPE after plasma treatment and modification by PEG/PEO were determined using atomic force microscopy (AFM). The AFM images were obtained by an MFP-3D AFM device (Oxford Instruments Asylum Research, Abingdon, Oxford, UK) using AC160TS probe (Veeco model, OLTESPA, Olympus, Tokyo, Japan), which is covered with a thin reflex aluminum coating in order to prevent the light directed from the microscope lens towards the sample surface being scattered or lost. Furthermore, AFM measurements were conducted under ambient conditions in the dynamic mode in air (AC mode) known also as tapping mode. This mode is preferred due to it overcoming technical problems related with friction, adhesion, electrostatic forces that may appear after a plasma treatment and cause image data to be distorted [41]. Moreover, AFM is an ideal tool to quantitatively measure the dimensional surface roughness in nano-scale and to visualize the surface nano-texture of the deposited film, via commonly parameter that describe the vertical dimensions of the surface, namely average surface roughness line (Ra). Ra is defined as an arithmetical mean height of a line of the irregularities in the direction perpendicular to the sample surface [42,43].

2.10. Surface Composition Evaluation Fourier-transform infrared spectroscopy (FTIR) was employed to identify the changes in chemical composition of the LDPE samples after plasma treatment and modiﬁcation process. FTIR spectra were recorded using (Spectrum 400, PerkinElmer, Waltham, MA, USA) equipped with a ZnSe crystal allowing the analysis of data from 1.66 µm of the penetration depth. This FTIR spectra were captured within a wavenumber range of 500–4000 cm−1 at spectral resolution of 4 cm−1 in the absorbance mode to collect 8 scans with the aim to obtain accurate FTIR spectra. The elemental and chemical compositions of the untreated, plasma-treated, and modi- ﬁed LDPE samples were evaluated using X-ray photoelectron spectroscopy (XPS) (Axis Ultra DLD, Kratos Analytical, Manchester, UK). XPS spectra were collected by irradiating a monoenergetic X-rays to the surface of a material, causing the emission of photoelectrons that are located within 10 nm from the underneath surface. Thus, the kinetic energy of the electrons emitted from each element present on the surface is analyzed, and the spectrum is obtained as a plot of the number of detected electrons per energy interval versus their kinetic energy. Furthermore, quantitative data were calculated based on the peaks formed by the individual elements according to the peak heights, areas, positions, and certain spectral features [44].

3. Results 3.1. Grafting Efficiency (GE) The changes in the grafting efficiency (GE) of corona-treated LDPE surfaces modified by PEG/PEO are shown in Figure4. The corona surface treatment had a significant effect on PEG/PEO grafting onto LDPE surfaces due to formation of radicals or reactive sites, which can react with PEG/PEO chains that are introduced into the surface. However, most probably, the grafting mechanism can be caused by an esterification process [45] as the result of interactions between the incorporated carboxylic groups in LDPE and Polymers 2021, 13, x FOR PEER REVIEW 8 of 19

Polymers 2021, 13, 1309 8 of 19 probably, the grafting mechanism can be caused by an esterification process [45] as the result of interactions between the incorporated carboxylic groups in LDPE and hydroxyl groupshydroxyl of groupsPEG/PEO of PEG/PEO[31] as was [31 confirmed] as was confirmed by FTIR measurements. by FTIR measurements. This leads This to an leads in- creaseto an increase in the mass in the of massthe modified of the modified specimen, specimen, thus increasing thus increasing GE [9]. It GE was [9 ].noted It was that noted GE increasedthat GE increased with increasing with increasing the PEG/PEO the PEG/PEO monomer monomer concentration concentration in the aqueous in the solution, aqueous duesolution, to incorporation due to incorporation of PEG/PEO of PEG/PEO chains onto chains LDPE onto surfaces. LDPE surfaces. This was This confirmed was confirmed by the presenceby the presence of the band of the at band 1100 atcm 1100−1 (C-O-C) cm−1 (C-O-C)in the FTIR in the spectr FTIRa [46]. spectra Moreover, [46]. Moreover, the highest the GEhighest was GEachieved was achieved for 5.0 wt.% for 5.0 PEO wt.% (300,000 PEO (300,000M)-g-LDPE M)-g-LDPE films preceded films preceded by 5 s of by plasma 5 s of surfaceplasma treatment, surface treatment, while GE while was GE approximately was approximately 0.6%. This 0.6%. can This be can explained be explained by PEO by (300,000PEO (300,000 M) having M) having ultrahigh ultrahigh molecular molecular weig weightshts that that can can create create a athicker thicker layer layer on on the LDPELDPE surface surface in in comparison comparison to to another another PEG being used.

0.6

0.5

0.4

0.3

0.2

Grafting efficiency, GE ( %) 0.1

0.0 G G O E E E P E P E P E PE P PE P PE .% D .% D DP D wt wt wt.% wt.% PEO .5 )-g-LD .5 )-g-LD .5 )-g-L 0 1 M 1 M 1 M 5. 10.0 wt.%0 PEG 10.0 wt.%0 PEG 0 0 0 0 0 0 0 (1000M)-g-L (1 (6000M)-g-L (6 0, 0 (3 (300,000M)-g-L

Figure 4. Effect of plasma treatment on grafting efficiency (GE) of PEG/PEO-g-LDPE. Figure 4. Effect of plasma treatment on grafting efficiency (GE) of PEG/PEO-g-LDPE. 3.2. Surface Wettability Analysis 3.2. SurfaceThe wettability Wettability property Analysis refers to the ability of a liquid to maintain in contact with a solidThe surface wettability and property it is characterized refers to the by ability the contact of a liquid angle to when maintain a droplet in contact of liquid with isa solidplaced surface on a flat,and horizontalit is characterized solid surface. by the contact In thiswork, angle threewhen testinga droplet liquids of liquid with is various placed onsurface a flat, tension horizontal and polarity,solid surface. namely In this water, work, formamide, three testing and ethyleneliquids with glycol, various were surface used to tensionstudy the and changes polarity, in namely the wettability water, formamide, after plasma and treatment ethylene and glycol, surface were modification used to study of theLDPE. changes As can in bethe seen wettability from Figure after5 a,plasma a dramatic treatment decrease and insurface the contact modification angle values of LDPE. was Asobserved can be withseen increasingfrom Figure the 5a, surface a dramatic treatment decrease time viain the corona contact discharge. angle values The maximum was ob- serveddecrease with in theincreasing contact the angle surface values treatmen was recordedt time via after corona 5 s ofdischarge. surface treatment,The maximum so it decreasecan be considered in the contact as the angle optimum values treatmentwas record timeed after for LDPE 5 s of surfaces.surface treatment, The contact so it angle can ◦ ◦ ◦ ◦ bevalues considered were decreased as the optimum from 72.3 treatmentto 57.5 timefor for water, LDPE from surfaces. 64.5 Theto 41.7 contactfor angle formamide, values ◦ ◦ wereand fromdecreased 57.8 fromto 29.1 72.3°for to ethylene 57.5° for glycolwater, correspondingfrom 64.5° to 41.7° to untreated for formamide, and 5 and s corona- from 57.8°treated to 29.1° LDPE for surfaces. ethylene However, glycol correspondi the relativelyng to lowuntreated values and of contact5 s corona-treated angles of testing LDPE surfaces.liquids for However, untreated the LDPE relatively were low probably values affected of contact by theangles processing of testing additives liquids for as wasun- treatedconfirmed LDPE by pre-sentwere probably oxygen-containing affected by groupsthe processing observed additives by XPS. Plasma as was treatment confirmed leads by pre-sentto surface oxygen-containing oxidation and introduction groups observed of new polarby XPS. functional Plasma groupstreatment such leads as C=O, to surface –OH, oxidationCOOH, C–O–C, and introduction into the LDPE of new surfaces polar func responsibletional groups for asuch wettability as C=O, –OH, increase COOH, [47]. C– In O–C,addition, into thethe effectLDPE of surfaces PEG/PEO responsible grafting onfor the a wettability wettability increase of LDPE [47]. was In studied addition, (Figure the 5b). A noticeable reduction in the contact angles of the PEG/PEO-g-LDPE surfaces were recorded compared to untreated LDPE surfaces. This can be explained by changes in Polymers 2021, 13, 1309 9 of 19

surface roughness as a result of PEG/PEO grafting [48]. However, a slight increase was observed in the contact angle values for all the PEG/PEO-g-LDPE in comparison with only corona-treated LDPE, as a result of chemical nature of PEG/PEO. Furthermore, it was revealed that as the concentration of PEG/PEO aqueous solutions increased, the contact angle became slightly lower for the same molecular weight, due to enriching the modified Polymers 2021, 13, x FOR PEER REVIEW 9 of 19 surface with PEG/PEO grafted on the LDPE surface, as well as hydrophilic properties of the PEG/PEO molecules themselves [49]. Figure6 shows the change in the surface free energy and the corresponding polar and effect of PEG/PEOdispersive grafting contributions on the ofwettability the untreated of LDPE and modifiedwas studied LDPE (Figure samples. 5b). ItA becamenotice- clear that able reductionthe surface in the contact free energies angles were of the significantly PEG/PEO-g-LDPE increased surfaces after surface were recorded treatment com- with corona pared to untreateddischarge LDPE from 30.3surfaces. mN/m This for can untreated be explained LDPE by to changes 42.6 mN/m in surface for corona-treated roughness LDPE as a resultdue of PEG/PEO to introduction grafting of [48]. characteristic However, polar a slight functional increase groups, was observed such as in C=O, the con- –OH, COOH, tact angleCOO–, values C–O–C,for all the to thePEG/PEO-g-LDPE substrate surface in[ 47comparison]. It was confirmed with only that corona-treated surface modification LDPE, as aof result LDPE of via chemical plasma-initiated nature of PEG/PEO. grafting of Furthermore, PEG/PEO contributed it was revealed to an improvementthat as the of the concentrationwettability of PEG/PEO properties aqueous of LDPE solutions surfaces, increased, as an the increase contact in angle both became the surface slightly energies and lower forpolarities the same were molecular observed weight, for all PEG/PEOdue to enriching used with the estimated modified percentages surface with of 31.3% and PEG/PEO63.0%, grafted respectively on the LDPE at surface, the minimum. as well Thisas hydrophilic indicated that properties the chemical of the characterPEG/PEO of grafted moleculesPEG/PEO themselves affected [49]. on the surface hydrophilicity of LDPE substrate [50].

80 Water Formamide Ethylene glycol

70 )

° 60

Contact angle ( 30

0 01357(untreated) Treatment time (sec.)

(a)

Figure 5. Cont. Polymers 2021, 13, x FOR PEER REVIEW 10 of 19 Polymers 2021, 13, 1309 10 of 19

70 Water Formamide Ethylene glycol

50 ) °

Contact angle ( Contact 20

E E E DP DP DP -L L -L -g -g- -g M) M) 0M) 000M)-g-LDPE 000 000M)-g-LDPE 000 1 6 (1 (6 0,00 G G 30

PE PE EO ( .% .% P % wt wt t. 1.5 wt.% PEG ( 0.0 1.5 wt.% PEG ( 0.0 1 1 .5 w 1 5.0 wt.% PEO (300,000M)-g-LDPE

(b) Polymers 2021, 13, x FOR PEER REVIEW 11 of 19

Figure 5. ContactFigure 5.angleContact of (a angle) LDPE of (samplesa) LDPE vs. samples corona-treatment vs. corona-treatment time, (b time,) PEG/PEO-g-LDPE. (b) PEG/PEO-g-LDPE.

Figure 6 shows the change in the surface free energy and the corresponding polar and dispersive 50contributions of the untreated and modified LDPE samples. It became clear ɣ ɣ d ɣ p that the surface free energies were significantly increased afters surfaces treatments with co- 45 rona discharge from 30.3 mN/m for untreated LDPE to 42.6 mN/m for corona-treated LDPE due to introduction40 of characteristic polar functional groups, such as C=O, –OH, COOH, COO–, C–O–C, to the substrate surface [47]. It was confirmed that surface modification of LDPE35 via plasma-initiated grafting of PEG/PEO contributed to an improve-

ment of the wettability30 properties of LDPE surfaces, as an increase in both the surface

energies and (mN/m) polarities were observed for all PEG/PEO used with estimated percentages p

of 31.3% ands 63.0%,25 respectively at the minimum. This indicated that the chemical char- ,ɣ

acter of graftedd PEG/PEO affected on the surface hydrophilicity of LDPE substrate [50]. s 20 ,ɣ s ɣ 15

0 E E G E DP DP P L E E E L % PEG % % PEG . t.% PEG DP t. DPE ted wt. w -LDP -LDP -L g g .5 )- trea 1 )-g-L )-g 1.5 wt.% PEO 5.0 wt.% PEO - 1.5 wtM)-g-LDPE10.0 w M 10.0 M M)- a 0 Untreated n ,000 (100 (1000M)-g-LDPE (6000 (6000 Coro (300 (300,000M

p d FigureFigure 6. 6. SurfaceSurface energy energy ( γs)) and and its components: polaritypolarity ( γ(γs),), andand dispersiondispersion ( γ(γs )) ofof LDPELDPE surfaces. surfaces.

3.3. Surface Morphology Analysis The changes in the morphological characteristics of the corona-treated (5 s) and PEG/PEO grafted LDPE samples were investigated by SEM analysis, as shown in Figure 7. The SEM image of untreated LDPE surface (Figure 7a) showed it excelled at low levels of surface roughness, while a noticeable increase in surface roughness was observed to the corona-treated LDPE samples as a consequence of surface ablation and etching processes (Figure 7b). In contrast, a slight increase was observed in surface roughness predominantly in the amorphous phase of PEG/PEO-g-LDPE surfaces affected by 5 s of continuous treatment by corona discharge (Figure 7c–h). However, it can be seen that the surface roughness of the PEG/PEO-g-LDPE samples is lower compared to the only corona-treated LDPE surface due to the formation of a compact PEG/PEO layer. The AFM measurements were performed in order to analyze detailed surface morphology/topography changes in the LDPE surface after plasma treatment and modification processes (Figure 8). The changes in the surface roughness were quantified by the surface roughness parameter (Ra). AFM images showed that the surface of the untreated LDPE is relatively smooth with low value of average roughness (Ra = 3.4 nm), as demonstrated in Figure 8a. Correspondingly, the surface treatment of the LDPE surface with 5 s of corona treatment led to an increase in the surface roughness, while Ra increased to 4.5 nm as a result of etching and ablation processes (Figure 8b). The morphologies/topographies of the corona-treated grafted PEG/PEO LDPE specimens were detected by AFM analysis, as evidenced in Figure 8c–h. It was observed that surface roughness of the PEG/PEO-g-LDPE specimens were greater than the untreated LDPE, due to the PEG/PEO graft on the modified surfaces. These results are consistent with contact angle measurements, because rougher surfaces reduce hydrophobicity and thus improve the wettability characteristics [48]. It was noticed that surface roughness decreased after PEG/PEO grafting onto the LDPE surfaces compared to corona-treated surfaces, due to a formation of PEG layer onto the LDPE surface. Moreover, it was found that increasing the concentration of the PEG/PEO-based aqueous solution resulted in less rough LDPE surface, due to high grafting density of PEG/PEO that leads to the creation of a thin layer. Moreover, it Polymers 2021, 13, 1309 11 of 19

3.3. Surface Morphology Analysis The changes in the morphological characteristics of the corona-treated (5 s) and PEG/PEO grafted LDPE samples were investigated by SEM analysis, as shown in Figure7. The SEM image of untreated LDPE surface (Figure7a) showed it excelled at low levels of surface roughness, while a noticeable increase in surface roughness was observed to the corona-treated LDPE samples as a consequence of surface ablation and etching processes (Figure7b). In contrast, a slight increase was observed in surface roughness predominantly in the amorphous phase of PEG/PEO-g-LDPE surfaces affected by 5 s of continuous treatment by corona discharge (Figure7c–h). However, it can be seen that the surface roughness of the PEG/PEO-g-LDPE samples is lower compared to the only corona-treated LDPE surface due to the formation of a compact PEG/PEO layer. The AFM measurements were performed in order to analyze detailed surface morphology/topography changes in the LDPE surface after plasma treatment and modification processes (Figure8). The changes in the surface roughness were quantified by the surface roughness parameter (Ra). AFM images showed that the surface of the untreated LDPE is relatively smooth with low value of average roughness (Ra = 3.4 nm), as demonstrated in Figure8a. Correspondingly, the surface treatment of the LDPE surface with 5 s of corona treatment led to an increase in the surface roughness, while Ra increased to 4.5 nm as a result of etching and ablation processes (Figure8b). The morphologies/topographies of the corona-treated grafted PEG/PEO LDPE specimens were detected by AFM analysis, as evidenced in Figure8c–h. It was observed that surface roughness of the PEG/PEO- g-LDPE specimens were greater than the untreated LDPE, due to the PEG/PEO graft on the modified surfaces. These results are consistent with contact angle measurements, because rougher surfaces reduce hydrophobicity and thus improve the wettability characteristics [48]. It was noticed that surface roughness decreased after PEG/PEO grafting onto the LDPE surfaces compared to corona-treated surfaces, due to a formation of PEG layer onto the LDPE surface. Moreover, it was found that increasing the concentration of the PEG/PEO-based aqueous solution resulted in less rough LDPE surface, due to high grafting density of PEG/PEO that leads to the creation of a thin layer. Moreover, it was observed that all LDPE films grafted by high concentrations of PEG/PEO had the same surface roughness (Ra = 3.6 nm)

3.4. Chemical Composition Investigation The FTIR analysis was used to identify the changes in the chemical composition of the LDPE surface after plasma treatment and surface modiﬁcation by PEG/PEO (Figure9). Generally, FTIR spectrum of untreated LDPE is characterized by characteristic absorption peaks, which coincide well with the relevant published literature, such as: out of phase and in-phase rock of the –CH2– at 720 cm-1 and 731 cm-1, weak asymmetric bending vibration of carbon-hydrogen bond (C–H) along the vertical axis (b-axis) of the LDPE chain at 1478 cm−1, asymmetric bending vibration of the CH3 groups along the horizontal axis (a- axis bend) at 1463 cm−1, as well as symmetric and asymmetric stretching vibrational bands that represent methylene group (C–H2) at 2848 cm−1 and 2916 cm−1, respectively [51]. The surface treatment by corona discharge led to signiﬁcant appearance of new absorption bands at 1750 cm−1 and 1110 cm−1 associated with stretching vibrations of C=O (COOH) and –O– respectively. In addition, the hydroxyl functional group (–OH) was represented by a less intense and broad absorption peak between 3500 cm−1 and 3180 cm−1. The emergence of these oxygen-containing functional groups in the LDPE surfaces was caused by an oxidation process. In addition, the FTIR spectra of PEG/PEO-g-LDPE surfaces exhibited a noticeable increase in the peak intensity corresponding to –O– compared with only corona-treated LDPE samples, while the peak intensity of C–H decreased. Moreover, the FTIR spectra clearly indicated the disappearance of the COOH-associated absorption bands in the PEG/PEO-g-LDPE samples, which were utilized in the grafting process. Polymers 2021, 13, x FOR PEER REVIEW 12 of 19

Polymers 2021, 13, 1309 12 of 19 was observed that all LDPE films grafted by high concentrations of PEG/PEO had the same surface roughness (Ra = 3.6 nm)

(a) (b)

(e) (f)

(g) (h)

FigureFigure 7. 7. SEMSEM micrographs of LDPE surface: surface: (a (a) )untreated, untreated, (b (b) )corona corona treated, treated, (c) ( c1.5) 1.5 wt.% wt.% PEG PEG (1000(1000 M)-g-LDPE, M)-g-LDPE, ((dd)) 10.0 wt.% wt.% PEG PEG (1000 (1000 M)-g-LDPE, M)-g-LDPE, (e ()e 1.5) 1.5 wt.% wt.% PEG PEG (6000 (6000 M)-g-LDPE, M)-g-LDPE, (f) 10.0 (f) 10.0wt.% wt.% PEG PEG (6000 (6000 M)-g-LDPE, M)-g-LDPE, (g) 1.5 (g) wt.%PEO 1.5 wt.%PEO (300,000 (300,000 M)-g-LDPE, M)-g-LDPE, (h) 5.0 (h) wt.% 5.0 wt.% PEO PEO (300,000 (300,000 M)- M)-g-LDPE.g-LDPE. Polymers 2021, 13, 1309 13 of 19 Polymers 2021, 13, x FOR PEER REVIEW 13 of 19

(a)

Ra = 3.4 nm

(b) Ra = 4.5 nm

(d) Ra = 3.6 nm

(e) Ra = 4.2 nm

(f) Ra = 3.6 nm

Figure 8. Cont. Polymers 2021, 13, 1309 14 of 19 Polymers 2021, 13, x FOR PEER REVIEW 14 of 19

(g) Ra = 4.1 nm

(h) Ra = 3.6 nm

Polymers 2021, 13, x FOR PEER REVIEW 15 of 19

was observed as the concentration of the PEG/PEO solutions increased and therefore at.% Figure 8. Atomic force microscopyof carbon (AFM) decreased, images indicati (3D Height,ng higher Amplitude, density line of profile) PEG/PEO with Ragrafted roughness on the parameter LDPE surface.of FigureLDPE 8. surface:Atomic ( forcea) untreated, microscopyFurthermore, (b) corona-treated, (AFM) a images decrease (c (3D) 1.5 in Height, wt.% the PEGnitrogen Amplitude, (1000 contM)-g-LDPE, lineent proﬁle) was (d observed) 10.0 with wt.% Ra roughnesson PEG the (1000 LDPEparameter M)-g-LDPE, surfaces of with LDPE(e) 1.5 surface: wt.% (PEGa) untreated, (6000 M)-g-LDPE,increasing (b) corona-treated, (f) the10.0 molecular wt.% (c) PEG 1.5 wt.% (6000 weight PEG M)-g-LDPE, of (1000 grafted M)-g-LDPE, (g) PEG 1.5 wt.%PEO chains (d) 10.0 (>1000)(300,000 wt.% PEG dueM)-g-LDPE, (1000 to a formation M)-g-LDPE, (h) 5.0 wt.% of (e a ) thin 1.5PEO wt.% (300,000 PEG (6000 M)-g-LDPE. M)-g-LDPE,coating (f) 10.0layer wt.% on PEGthe LDPE (6000 M)-g-LDPE,surface, which (g) 1.5 hind wt.%PEOers the (300,000 detection M)-g-LDPE, of the internal (h) 5.0 wt.%nitrogen PEO (300,000 M)-g-LDPE. element [36]. 3.4. Chemical Composition Investigation The FTIR analysisval. asym.was used to identify the changes in the chemical composition of - CH2 - the LDPE surface afterval. plasma sym. treatment and surface modification by PEG/PEO (Figure 9). Generally, FTIR spectrum- CH2 - of untreated LDPE is characterized by characteristic absorption peaks, which coincide well with the relevant published literature, such as: out of phase and in-phase rock of the –CH2– at 720 cm‒1 and 731 cm‒1, weak asymmetric bending vibration of carbon-hydrogen bond (C–H) along the vertical axis (b-axis) of the LDPE chain at 1478 cm−1, asymmetric bending vibration of the CH3 groups along the horizontal def. axis (a-axis bend) at 1463 cm−1, as well as symmetricdef. asym. and asymmetric stretching vibrational - CH2 - bands that represent methylene group (C–H2)- CH3 at 2848 cm−1 and 2916 cm−1, respectively [51]. The surface treatment by corona discharge led to significant appearance of new ab- def. asym. −1 −1 -O- sorption bands at 1750 cm and 1110 cm associated- CH with stretching vibrations of C=O - OH C=O (COOH)(h) and –O– respectively. In addition, the hydroxyl functional group (–OH) was rep- (g)

Absorbance (a.u) −1 −1 resented(f) by a less intense and broad absorption peak between 3500 cm and 3180 cm . (e) The (d)emergence of these oxygen-containing functional groups in the LDPE surfaces was caused(c) by an oxidation process. In addition, the FTIR spectra of PEG/PEO-g-LDPE sur- (b) faces(a) exhibited a noticeable increase in the peak intensity corresponding to –O– compared with only corona-treated LDPE samples, while the peak intensity of C–H decreased. 4000 3500 3000 2500 2000 1500 1000 500 Moreover, the FTIR spectra clearly indicated the disappearance of the COOH-associated Wavenumbers (cm-1) absorption bands in the PEG/PEO-g-LDPE samples, which were utilized in the grafting process. Figure 9. FTIR spectra of LDPE surfaces: ((aa)) untreated,untreated, ((bb)) corona-treated,corona-treated, ( c(c)) 1.5 1.5 wt.%PEG wt.%PEG (1000 (1000 M)- The XPS technique provides quantitative information about the elemental composi- M)-g-LDPE,g-LDPE, (d) 10.0(d) 10.0 wt.%PEG wt.%PEG (1000 (1000 M)-g-LDPE, M)-g-LDPE, (e) 1.5(e) 1.5 wt.%PEG wt.%PEG (6000 (6000 M)-g-LDPE, M)-g-LDPE, (f) 10.0(f) 10.0 wt.%PEG tions of the untreated, corona-treated, and modified LDPE surfaces as seen in Figure 10 (6000wt.%PEG M)-g-LDPE, (6000 M)-g-LDPE, (g) 1.5 wt.%PEO (g) 1.5 (300,000wt.%PEO M)-g-LDPE, (300,000 M)-g-LDPE, (h) 5.0 wt.%PEO (h) 5.0 (300,000wt.%PEO M)-g-LDPE. (300,000 M)-g- and Table 3. As can be seen, there are two characteristic XPS peaks corresponding to the LDPE. C1sThe and XPS O1s technique at binding provides energy values quantitative of 284.8 information and 532.8 eV, about respectively. the elemental A slightly composi- in- crease in the oxygen content was observed after corona treatment, while at.% of O1s in- tions of the untreated, corona-treated, and modiﬁedC 1s LDPE surfaces as seen in Figure 10 and Tablecreased3. As from can 8.6 be seen,to 11.3% there for areuntreated two characteristic and corona-treated XPS peaks LDPE, corresponding respectively, todue the to C1sthe enrichingb of the surface with oxygen-containing functional species. However, the pres- and O1sc at binding energy values of 284.8 and 532.8 eV, respectively. A slightly increase in ence ofd oxygen species in the untreated LDPE structure may be related to the processing the oxygene content was observed after corona treatment, while at.% of O1s increased from 8.6additives. to 11.3%f After for untreated PEG/PEO and grafting corona-treated onto LDPE LDPE,surfaces respectively, via plasma treatment, due to the it enriching was found of thethat surface the at.% with of carbon oxygen-containing element increased functional compared species. to untreated However, LDPE the presenceas results ofof oxygenhigher speciescarbon in to the oxygen untreated ratio LDPEin PEG/PEO. structure Furtherm may beore, related a slight to theincrease processing in the additives.oxygen content After

PEG/PEO grafting onto LDPE surfacesO 1s via plasma treatment, it was found that the at.%

F 1s N 1s

(h) Intensity (cps) (g) (f) (e) (d) (c) (b) (a)

1200 1000 800 600 400 200 0

Binding Energy (eV) . Figure 10. XPS spectra of LDPE surfaces.

Polymers 2021, 13, x FOR PEER REVIEW 15 of 19

was observed as the concentration of the PEG/PEO solutions increased and therefore at.% of carbon decreased, indicating higher density of PEG/PEO grafted on the LDPE surface. Furthermore, a decrease in the nitrogen content was observed on the LDPE surfaces with increasing the molecular weight of grafted PEG chains (>1000) due to a formation of a thin coating layer on the LDPE surface, which hinders the detection of the internal nitrogen element [36].

val. asym. - CH2 - val. sym. - CH2 -

def. def. asym. - CH - - CH3 2

def. asym. - CH -O- - OH C=O (h) Polymers 2021, 13, 1309 (g) 15 of 19 Absorbance (a.u) (f) (e) (d) (c) (b) of carbon(a) element increased compared to untreated LDPE as results of higher carbon

to oxygen4000 ratio 3500 in PEG/PEO.3000 2500 Furthermore, 2000 1500 a slight 1000 increase 500 in the oxygen content was observed as the concentration of the PEG/PEO solutions increased and therefore at.% of Wavenumbers (cm-1) carbon decreased, indicating higher density of PEG/PEO grafted on the LDPE surface. FigureFurthermore, 9. FTIR spectra a decrease of LDPE in the surfaces: nitrogen (a) untreated, content was (b) observedcorona-treated, on the (c) LDPE1.5 wt.%PEG surfaces (1000 with M)-g-LDPE,increasing the(d) molecular10.0 wt.%PEG weight (1000 of M)-g-LDPE, grafted PEG (e) 1.5 chains wt.%PEG (>1000) (6000 due M)-g-LDPE, to a formation (f) 10.0 of a thin wt.%PEGcoating layer (6000 onM)-g-LDPE, the LDPE (g) surface, 1.5 wt.%PEO which (300,000 hinders M)-g-LDPE, the detection (h) 5.0 of wt.%PEO the internal (300,000 nitrogen M)-g- LDPE.element [36].

C 1s

b c d e f

O 1s

F 1s N 1s

(h) Intensity (cps) (g) (f) (e) (d) (c) (b) (a)

1200 1000 800 600 400 200 0

Binding Energy (eV) . Figure 10. XPSXPS spectra spectra of of LDPE surfaces.

Table 3. Elemental composition of LDPE surfaces by XPS analysis.

Element, Atomic Conc. (at. %) Samples C 1s O 1s N 1s (a) Untreated-LDPE 91.2 8.6 0.2 (b) Corona-treated LDPE 88.5 11.3 0.2 (c) 1.5 wt.% PEG (1000M)-g-LDPE 95.2 4.6 0.2 (d) 10.0 wt.% PEG (1000M)-g-LDPE 93.6 6.1 0.3 (e) 1.5 wt.% PEG (6000M)-g-LDPE 95.3 4.5 0.00 (f) 10.0 wt.% PEG (6000M)-g-LDPE 92.3 7.5 0.04 (g) 1.5 wt.% PEO (300,000M)-g-LDPE 95.1 5.2 0.03 (h) 5.0 wt.% PEO (300,000M)-g-LDPE 90.6 9.2 0.0

3.5. Adhesive Strength Measurements The adhesive strength of the untreated, plasma treated and PEG/PEO-g-LDPE laminates with Al were analyzed using peeling resistance measurements, as shown in Figure 11. It can be seen that the peel resistance of the untreated LDPE/Al joints were nearly 3 N/m due to poor adhesion between the LDPE and Al laminate components. This can be in- terpreted to hydrophobic nature and low wettability of pristine LDPE surfaces. The peel resistance of LDPE/Al laminates remarkably increased (≈62.5 N/m) after plasma treatment using corona discharge as a result of improved wettability and roughness. In addition, it was observed that LDPE/Al adhesion joints prepared using the PEG/PEO-g-LDPE samples had a notable increase in peel resistance compared to untreated LDPE. This increase in peel resistance was mainly due to increase in the wettability and surface roughness Polymers 2021, 13, 1309 16 of 19

caused by the incorporation of oxygen-rich functional. In addition, the highest peeling resistance values were recorded at high concentrations of PEG/PEO aqueous solutions, which was consistent with the surface wettability results obtained from the contact angle measurements of PEG/PEO-g-LDPE surfaces. The maximum peel resistance (163.0 N/m) of LDPE/Al adhesive joint was observed for LDPE/10.0% PEG (6000 M), which suggested as the optimum value. In contrast, the corona-treated LDPE grafted by PEO (300,000 M) surfaces exhibited the lowest adhesion values compared to other PEG used. This might be because as the molecular weight of PEG increased, the mole fraction of the reactive -OH groups decreased, to the point where the active bonding sites available on the LDPE surface are saturated and no more extend. Thus affecting on their surface hydrophilicity and thus led to reduced adherence to Al [52]. Moreover, work of adhesion (W12) for the LDPE/Al adhesive joints were calculated using surface free energy and its components. It was found that W12 values showed similar behavior as the peel resistance for all conditions used. The reason for the lower values of W12 than peeling resistance of LDPE/Al adhesion joint was the fact that W12 counts with inﬁnite slow peeling rate, while crosshead speed during the peel resistance measurement was 10 mm/min. However, it was found that all modiﬁed PEG/PEO-g- LDPE surfaces had higher values of W12 compared to untreated Polymers 2021, 13, x FOR PEER REVIEW 17 of 19 and corona-treated LDPE surfaces because of higher values of the polar component of the surface energy resulted from the improved wettability.

180 74 Peel resistance Work of adhesion, W12

170 73

160 72 90

71 x1000 (N/m) 12

80 Corona-treated 70 70

Corona-treated 69 Peel resistance (N/m) resistance Peel 60 Untreated 10 56 Work of adhesion, W Work of adhesion,

Untreated 0 55

G G O O EG E EG E P E E P E E PE E PE E % % P DP % DP % P DP % DP % DP LDP wt. - wt. -L wt. -L wt. -L wt. -L wt. -L 5 )-g 0 )-g .5 )-g .0 )-g .5 )-g .0 )-g 1. M M 1 M 0 1 M 5 M 0 10. 0 0 1 0 0 0 0 0 0 0 000M ,0 ,0 0 0 (10 (10 (60 (6 0 0 (3 (3

FigureFigure 11. 11. EffectEffect of of corona corona treatment treatment on on the the peel peel resi resistancestance and and work work of ofadhesion adhesion of ofPEG/PEO-g- PEG/PEO-g- LDPELDPE adhesive adhesive joint joint with with Al. Al. 4. Conclusions 4. Conclusions In this work, the surface characteristics of low-density polyethylene (LDPE) were In this work, the surface characteristics of low-density polyethylene (LDPE) were en- enhanced using plasma-initiated grafting of different molecular weight polyethylene glycol hanced using plasma-initiated grafting of different molecular weight polyethylene glycol or polyethylene oxide (PEG/PEO) onto LDPE surfaces in order to improve the adhesion to or polyethylene oxide (PEG/PEO) onto LDPE surfaces in order to improve the adhesion aluminum (Al) for industrial purposes. This surface modiﬁcation improved the surface to aluminum (Al) for industrial purposes. This surface modification improved the surface wettability as was conﬁrmed by a decrease in the contact angles, and thus increased both wettability as was confirmed by a decrease in the contact angles, and thus increased both the surface free energy and its polar component as a consequence of the change in the the surface free energy and its polar component as a consequence of the change in the chemical composition of the modified LDPE surfaces. Moreover, the chemical composition analyses confirmed the presence of PEG/PEO on the corona-treated LDPE surface through esterification process. This led to considerable enhancement in the interfacial adhesion between LDPE and Al compared to the untreated and corona-treated surfaces. It was found that the adhesion strength between LDPE and Al surfaces were achieved at high concentrations of aqueous solutions containing PEG/PEO compounds. This could be due to improved wettability of the treated surfaces as confirmed by contact angle measurements as well as results obtained from typical surface analyzes. However, the highest adhesion in the LDPE/Al laminate was achieved by grafting with a 10% PEG (6000 M) aqueous solution onto 5 s corona-treated LDPE surface, where the peel resistance increased by approximately 54 times and 2.6 times compared to the peel resistance of untreated and corona-treated LDPE surfaces, respectively.

chemical composition of the modified LDPE surfaces. Moreover, the chemical composition analyses confirmed the presence of PEG/PEO on the corona-treated LDPE surface through esterification process. This led to considerable enhancement in the interfacial adhesion between LDPE and Al compared to the untreated and corona-treated surfaces. It was found that the adhesion strength between LDPE and Al surfaces were achieved at high concentrations of aqueous solutions containing PEG/PEO compounds. This could be due to improved wettability of the treated surfaces as confirmed by contact angle measurements as well as results obtained from typical surface analyzes. However, the highest adhesion in the LDPE/Al laminate was achieved by grafting with a 10% PEG (6000 M) aqueous solution onto 5 s corona-treated LDPE surface, where the peel resistance increased by approximately 54 times and 2.6 times compared to the peel resistance of untreated and corona-treated LDPE surfaces, respectively.

Author Contributions: Conceptualization, A.P., I.K., M.O., and S.K.K.; methodology, A.P. and I.K.; validation, A.P.; formal analysis, T.A.A.-G. and A.P.; investigation, T.A.A.-G. and A.P.; resources, I.K., M.O. and S.K.K.; data curation, A.P.; writing—original draft preparation, T.A.A.-G.; writing—review and editing, A.P.; visualization, T.A.A.-G.; supervision, A.P.; project administration, I.K.; funding acquisition, I.K. All authors have read and agreed to the published version of the manuscript. Funding: This publication was made possible by Award QUEX-CAM-QAPCO-20/21 from Qatar Petrochemical Company and NPRP13S-0127-200177 from the Qatar National Research Fund (a mem- ber of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. Acknowledgments: In this section, you can acknowledge any support given which is not covered by the author contribution or funding sections. This may include administrative and technical support, or donations in kind (e.g., materials used for experiments). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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